[3.6] bpo-32614: Modify re examples to use a raw string to prevent wa… …rning ...

 [3.6] bpo-32614: Modify re examples to use a raw string to prevent wa…  …rning (GH-5265) (GH-5500)

Modify RE examples in documentation to use raw strings to prevent DeprecationWarning.
Add text to REGEX HOWTO to highlight the deprecation.  Approved by Serhiy Storchaka.

(cherry picked from commit 66771422d0541289d0b1287bc3c28e8b5609f6b4)