Add change note to docs. [skip ci]

docs/api/gevent.socket.rst

@@ -45,6 +45,12 @@ Python 2 and Python 3, respectively.
     The cooperative socket object. See the version documentation for
     specifics.
 
+    .. versionchanged:: 20.12.0
+
+       Socket objects no longer have a ``__dict__`` or accept
+       arbitrary instance variables. Previously they did, but
+       standard library sockets never have.
+
 
 .. toctree::
 

src/gevent/_util.py

@@ -311,6 +311,8 @@ def prereleaser_middle(data): # pragma: no cover
         new_version_bytes = new_version
     new_version_bytes = new_version.encode('ascii')
     replacement = br'.. \1:: %s' % (new_version_bytes,)
+    # TODO: This should also look in the docs/ directory at
+    # *.rst
     for path, _ in modules.walk_modules(
             # Start here
             basedir=os.path.join(data['reporoot'], 'src', 'gevent'),