[3.7] bpo-38294: Add list of no-longer-escaped chars to re.escape documentation....

[3.7] bpo-38294: Add list of no-longer-escaped chars to re.escape documentation. (GH-16442) (GH-16647)

Prior to 3.7, re.escape escaped many characters that don't have
special meaning in Python, but that use to require escaping in other
tools and languages. This commit aims to make it clear which characters
were, but are no longer escaped.
(cherry picked from commit 15ae75d660befe643ed42eb2707a557cea97256c)