[docs] base64 "legacy" functions violate RFC 3548 (issue 1753718)

[vadmium+py at gmail.com]

https://bugs.python.org/review/1753718/diff/16149/Doc/library/base64.rst
File Doc/library/base64.rst (right):

https://bugs.python.org/review/1753718/diff/16149/Doc/library/base64.rst#newcode197
Doc/library/base64.rst:197: If *pad* is true, the input is padded with
"\\0" so its length is a
padded with b"\\0"

https://bugs.python.org/review/1753718/diff/16149/Doc/library/base64.rst#newcode198
Doc/library/base64.rst:198: multiple of 4 characters before encoding.
four bytes

https://bugs.python.org/review/1753718/