docs: document the "internal API" with reference & howto

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diff --git a/docs/api/api.rst b/docs/api/api.rst
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index 00000000..55642ef6
--- /dev/null
+++ b/docs/api/api.rst
@@ -0,0 +1,88 @@
+.. _internal-api:
+
+Internal "API"
+==============
+
+Slixmpp has a generic API registry that can be used by its plugins to allow
+access control, redefinition of behaviour, without having to inherit from the
+plugin or do more dark magic.
+
+The idea is that each api call can be replaced, most of them use a form
+of in-memory storage that can be, for example, replaced with database
+or file-based storaged.
+
+
+Each plugin is assigned an API proxy bound to itself, but only a few make use
+of it.
+
+See also :ref:`api-simple-tuto`.
+
+Description of a generic API call
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+.. code-block:: python
+
+    def get_toto(jid, node, ifrom, args):
+        return 'toto'
+
+    self.xmpp.plugin['xep_XXXX'].api.register(handler, 'get_toto')
+
+Each API call will receive 4 parameters (which can be ``None`` if data
+is not relevant to the operation), which are ``jid`` (``Optional[JID]``),
+``node`` (``Optional[str]``), ``ifrom`` (``Optional[JID]``), and ``args``
+(``Any``).
+
+- ``jid``, if relevant, represents the JID targeted by that operation
+- ``node``, if relevant is an arbitrary string, but was thought for, e.g.,
+  a pubsub or disco node.
+- ``ifrom``, if relevant, is the JID the event is coming from.
+- ``args`` is the event-specific data passed on by the plugin, often a dict
+  of arguments (can be None as well).
+
+.. note::
+    Since 1.8.0, API calls can be coroutines.
+
+
+Handler hierarchy
+~~~~~~~~~~~~~~~~~
+
+The ``self.api.register()`` signature is as follows:
+
+.. code-block:: python
+
+    def register(handler, op, jid=None, node=None, dedfault=False):
+        pass
+
+As you can see, :meth:`~.APIRegistry.register` takes an additional ctype
+parameter, but the :class:`~.APIWrapper` takes care of that for us (in most
+cases, it is the name of the XEP plugin, such as ``'xep_0XXX'``).
+
+When you register a handler, you register it for an ``op``, for **operation**.
+For example, ``get_vcard``.
+
+``handler`` and ``op`` are the only two required parameters (and in many cases,
+all you will ever need). You can, however, go further and register handlers
+for specific values of the ``jid`` and ``node`` parameters of the calls.
+
+The priority of the execution of handlers is as follows:
+
+- Check if a handler for both values of ``node`` and ``jid`` has been defined
+- If not found, check if a handler for this value of ``jid`` has been defined
+- If not found, check if a handler for this value of ``node`` has been defined
+- If still not found, get the global handler (no parameter registered)
+
+
+Raw documentation
+~~~~~~~~~~~~~~~~~
+
+This documentation is provided for reference, but :meth:`~.APIRegistry.register`
+should be all you need.
+
+
+.. module:: slixmpp.api
+
+.. autoclass:: APIRegistry
+    :members:
+
+.. autoclass:: APIWrapper
+
diff --git a/docs/api/index.rst b/docs/api/index.rst
index aefa0f56..e9aa6264 100644
--- a/docs/api/index.rst
+++ b/docs/api/index.rst
@@ -14,3 +14,4 @@ API Reference
     xmlstream/matcher
     xmlstream/xmlstream
     xmlstream/tostring
+    api
diff --git a/docs/howto/index.rst b/docs/howto/index.rst
index b05dc499..e4dee4d7 100644
--- a/docs/howto/index.rst
+++ b/docs/howto/index.rst
@@ -6,6 +6,7 @@ Tutorials, FAQs, and How To Guides
 
     stanzas
     create_plugin
+    internal_api
     features
     sasl
     handlersmatchers
diff --git a/docs/howto/internal_api.rst b/docs/howto/internal_api.rst
new file mode 100644
index 00000000..003225c4
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+++ b/docs/howto/internal_api.rst
@@ -0,0 +1,94 @@
+.. _api-simple-tuto:
+
+Flexible internal API usage
+===========================
+
+The :ref:`internal-api` in slixmpp is used to override behavior or simply
+to override the default, in-memory storage backend with something persistent.
+
+
+We will use the XEP-0231 (Bits of Binary) plugin as an example here to show
+very basic functionality. Its API reference is in the plugin documentation:
+:ref:`api-0231`.
+
+Let us assume we want to keep each bit of binary in a file named with its
+content-id, with all metadata.
+
+First, we have to load the plugin:
+
+.. code-block:: python
+
+    from slixmpp import ClientXMPP
+    xmpp = ClientXMPP(...)
+    xmpp.register_plugin('xep_0231')
+
+This enables the default, in-memory storage.
+
+We have 3 methods to override to provide similar functionality and keep things
+coherent.
+
+Here is a class implementing very basic file storage for BoB:
+
+.. code-block:: python
+
+    from slixmpp.plugins.xep_0231 import BitsOfBinary
+    from os import makedirs, remove
+    from os.path import join, exists
+    import base64
+    import json
+
+    class BobLoader:
+        def __init__(self, directory):
+            makedirs(directory, exist_ok=True)
+            self.dir = directory
+
+        def set_bob(self, jid=None, node=None, ifrom=None, args=None):
+            payload = {
+                'data': base64.b64encode(args['data']).decode(),
+                'type': args['type'],
+                'cid': args['cid'],
+                'max_age': args['max_age']
+            }
+            with open(join(self.dir, args['cid']), 'w') as fd:
+                fd.write(json.dumps(payload))
+
+        def get_bob(self, jid=None, node=None, ifrom=None, args=None):
+            with open(join(self.dir, args), 'r') as fd:
+                payload = json.loads(fd.read())
+            bob = BitsOfBinary()
+            bob['data'] = base64.b64decode(payload['data'])
+            bob['type'] = payload['type']
+            bob['max_age'] = payload['max_age']
+            bob['cid'] = payload['cid']
+            return bob
+
+        def del_bob(self, jid=None, node=None, ifrom=None, args=None):
+            path = join(self.dir, args)
+            if exists(path):
+                remove(path)
+
+Now we need to replace the default handler with ours:
+
+.. code-block:: python
+
+
+    bobhandler = BobLoader('/tmp/bobcache')
+    xmpp.plugin['xep_0231'].api.register(bobhandler.set_bob, 'set_bob')
+    xmpp.plugin['xep_0231'].api.register(bobhandler.get_bob, 'get_bob')
+    xmpp.plugin['xep_0231'].api.register(bobhandler.del_bob, 'del_bob')
+
+
+And that’s it, the BoB storage is now made of JSON files living in a
+directory (``/tmp/bobcache`` here).
+
+
+To check that everything works, you can do the following:
+
+.. code-block:: python
+
+    cid = await xmpp.plugin['xep_0231'].set_bob(b'coucou', 'text/plain')
+    # A new bob file should appear
+    content = await xmpp.plugin['xep_0231'].get_bob(cid=cid)
+    assert content['bob']['data'] == b'coucou'
+
+A file should have been created in that directory.