This note incorrectly stated that "Base64 has an expansion factor of 6
to 4" (it is actually 4 to 3). It was decided to remove the note.

Allow developers to not have to either test on N Python versions or
looked through multiple versions of the docs to know whether they can
easily update.

Reformat header above separator line (added if missing) to a common format.
Patch by Yoni Lavi.

* There are only two base-64 alphabets defined by the RFCs, not three
* Due to the internal translation, plus (+) and slash (/) are never discarded
* standard_ and urlsafe_b64decode() discard characters as well

Also update the doc strings to clarify data types, based on revision
92760d2edc9e, correct the exception raised by b16decode(), and correct the
parameter name for the base-85 functions.

Patch includes contributions by Isobel Hooper, incorporating suggestions from
Paul Winkler.  Reviewed by Martin Panter.

In addition to accepting the corrections for the RFC compliance wording, I
went through and corrected all the argument and return types, and made the
pattern of how the arguments and return types are documented consistent.
So, this patch also addresses #20782, though I had forgotten about that issue
and its patch.