[docs] Inconsistency between b32decode() documentation, docstring and code (issue 18011)

[ethan at stoneleaf.us]

http://bugs.python.org/review/18011/diff/8191/Lib/base64.py
File Lib/base64.py (right):

http://bugs.python.org/review/18011/diff/8191/Lib/base64.py#newcode225
Lib/base64.py:225: raise binascii.Error('Non-base32 digit found')
Should we add a `from None` here to suppress the double exception
traceback?

http://bugs.python.org/review/18011/