gh-161311: Move abstract methods clarification to introductory text

Added a note about the 'Abstract Methods' column in the collections ABCs documentation.

Author: Bayel1602

Doc/library/collections.abc.rst

@@ -111,11 +111,13 @@ Collections Abstract Base Classes
 
 The collections module offers the following :term:`ABCs <abstract base class>`:
 
+The "Abstract Methods" column lists all required abstract methods,
+including those inherited from parent classes.
+
 .. tabularcolumns:: |l|L|L|L|
 
 ============================== ====================== ======================= ====================================================
 ABC                            Inherits from          Abstract Methods        Mixin Methods
-                                                      (including inherited)
 ============================== ====================== ======================= ====================================================
 :class:`Container` [1]_                               ``__contains__``
 :class:`Hashable` [1]_                                ``__hash__``