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

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+.. _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
+