bpo-44690: Adopt binacii.a2b_base64's strict mode in base64.b64decode (GH-27272)

* Use binascii.a2b_base64 to validate b64decode input.
   This change leads to exception messages changes (mostly).
* Added more information to docstring of b64decode
* Added a reference to binascii.a2b_base64 in the docs