gh-151950: Document async.BaseEventLoop and fix asyncio-extending.rst

Author: sobolevn

Doc/library/asyncio-eventloop.rst

@@ -1917,6 +1917,16 @@ By default asyncio is configured to use :class:`EventLoop`.
    methods that an alternative implementation of ``AbstractEventLoop``
    should have defined.
 
+.. class:: BaseEventLoop
+
+   Abstract base class for asyncio-compliant event loops.
+   Inherits from :class:`AbstractEventLoop` directly.
+
+   Has many methods of ``AbstractEventLoop`` implemented, so end users
+   would have less methods to implement themselfs.
+   But, instead forces to implement some internal protected API methods.
+   See :ref:`writting_custom_event_loop` for more information.
+
 
 Examples
 ========

Doc/library/asyncio-extending.rst

@@ -15,11 +15,14 @@ classes. Asyncio has helpers that could be used to simplify this task.
    in *internal* part of API.
 
 
+.. _writting_custom_event_loop:
+
 Writing a Custom Event Loop
 ===========================
 
-:class:`asyncio.AbstractEventLoop` declares very many methods.  Implementing all them
-from scratch is a tedious job.
+:class:`asyncio.AbstractEventLoop` declares very many methods.
+See :ref:`asyncio-event-loop-methods` for the full list.
+Implementing all them from scratch is a tedious job.
 
 A loop can get many common methods implementation for free by inheriting from
 :class:`asyncio.BaseEventLoop`.

Doc/tools/.nitignore

@@ -6,7 +6,6 @@ Doc/c-api/init_config.rst
 Doc/c-api/intro.rst
 Doc/c-api/stable.rst
 Doc/library/ast.rst
-Doc/library/asyncio-extending.rst
 Doc/library/email.charset.rst
 Doc/library/email.parser.rst
 Doc/library/http.cookiejar.rst