More complete documentation of event loops and policies.

Documented the AbstractEventLoopPolicy interface explicitly and explained the
relation between the global loop & policy access functions. Added an initial
section that explains the connections in general terms. Documented missing XXX
methods.

Doc/library/asyncio-eventloop.rst

@@ -13,44 +13,83 @@ It provides multiple facilities, amongst which:
 * Creating client and server :ref:`transports <asyncio-transport>` for various
   kinds of communication.
 
-* Launching subprocesses and the associated :ref:`transports <asyncio-transport>`
-  for communication with an external program.
+* Launching subprocesses and the associated :ref:`transports
+  <asyncio-transport>` for communication with an external program.
 
 * Delegating costly function calls to a pool of threads.
 
+Event loop policies and the default policy
+------------------------------------------
+
+Event loop management is abstracted with a *policy* pattern, to provide maximal
+flexibility for custom platforms and frameworks. Throughout the execution of a
+process, a single global policy object manages the event loops available to the
+process based on the calling context. A policy is an object implementing the
+:class:`AbstractEventLoopPolicy` interface.
+
+For most users of :mod:`asyncio`, policies never have to be dealt with
+explicitly, since the default global policy is sufficient.
+
+The default policy defines context as the current thread, and manages an event
+loop per thread that interacts with :mod:`asyncio`. The module-level functions
+:func:`get_event_loop` and :func:`set_event_loop` provide convenient access to
+event loops managed by the default policy.
+
 Event loop functions
 --------------------
 
-The easiest way to get an event loop is to call the :func:`get_event_loop`
-function.
+The following functions are convenient shortcuts to accessing the methods of the
+global policy. Note that this provides access to the default policy, unless an
+alternative policy was set by calling :func:`set_event_loop_policy` earlier in
+the execution of the process.
 
 .. function:: get_event_loop()
 
-   Get the event loop for current context. Returns an event loop object
-   implementing :class:`BaseEventLoop` interface, or raises an exception in case no
-   event loop has been set for the current context and the current policy does
-   not specify to create one. It should never return ``None``.
+   Equivalent to calling ``get_event_loop_policy().get_event_loop()``.
 
 .. function:: set_event_loop(loop)
 
-   XXX
+   Equivalent to calling ``get_event_loop_policy().set_event_loop(loop)``.
 
 .. function:: new_event_loop()
 
-   XXX
+   Equivalent to calling ``get_event_loop_policy().new_event_loop()``.
 
+Event loop policy interface
+---------------------------
 
-Event loop policy
------------------
+An event loop policy must implement the following interface:
+
+.. class:: AbstractEventLoopPolicy
+
+   .. method:: get_event_loop()
+
+   Get the event loop for current context. Returns an event loop object
+   implementing :class:`BaseEventLoop` interface, or raises an exception in case
+   no event loop has been set for the current context and the current policy
+   does not specify to create one. It should never return ``None``.
+
+   .. method:: set_event_loop(loop)
+
+   Set the event loop of the current context to *loop*.
+
+   .. method:: new_event_loop()
+
+   Create and return a new event loop object according to this policy's rules.
+   If there's need to set this loop as the event loop of the current context,
+   :meth`set_event_loop` must be called explicitly.
+
+Access to the global loop policy
+--------------------------------
 
 .. function:: get_event_loop_policy()
 
-   XXX
+   Get the current event loop policy.
 
 .. function:: set_event_loop_policy(policy)
 
-   XXX
-
+   Set the current event loop policy. If *policy* is ``None``, the default
+   policy is restored.
 
 Run an event loop
 -----------------