[3.6] bpo-30085: Improve documentation for operator (GH-1171) (#3736)

The dunderless functions are preferred; dunder are retained for back compatilibity.
Patch by Sanket Dasgupta.
(cherry picked from commit 5b9299d8)

Doc/library/operator.rst

@@ -17,9 +17,10 @@
 
 The :mod:`operator` module exports a set of efficient functions corresponding to
 the intrinsic operators of Python.  For example, ``operator.add(x, y)`` is
-equivalent to the expression ``x+y``.  The function names are those used for
-special class methods; variants without leading and trailing ``__`` are also
-provided for convenience.
+equivalent to the expression ``x+y``. Many function names are those used for
+special methods, without the double underscores.  For backward compatibility,
+many of these have a variant with the double underscores kept. The variants
+without the double underscores are preferred for clarity.
 
 The functions fall into categories that perform object comparisons, logical
 operations, mathematical operations and sequence operations.

Misc/NEWS.d/next/Documentation/2017-09-14-18-44-50.bpo-30085.0J9w-u.rst

+The operator functions without double underscores are preferred for clarity.
+The one with underscores are only kept for back-compatibility.