Issue #21545: Add .pop example and tweak comment about pure mutation methods.

Patch prepared by David Harrigan.

Author: terryjreedy

Doc/tutorial/datastructures.rst

@@ -99,6 +99,15 @@ An example that uses most of the list methods::
    >>> a.sort()
    >>> a
    [-1, 1, 66.25, 333, 333, 1234.5]
+   >>> a.pop()
+   1234.5
+   >>> a
+   [-1, 1, 66.25, 333, 333]
+
+You might have noticed that methods like ``insert``, ``remove`` or ``sort`` that
+only modify the list have no return value printed -- they return the default
+``None``. [1]_  This is a design principle for all mutable data structures in
+Python.
 
 
 .. _tut-lists-as-stacks:

Misc/ACKS

@@ -506,6 +506,7 @@ Barry Hantman
 Lynda Hardman
 Derek Harland
 Jason Harper
+David Harrigan
 Brian Harring
 Jonathan Hartley
 Travis B. Hartwell