bpo-39416: Document some restrictions on the default string representations of numeri...

bpo-39416: Document some restrictions on the default string representations of numeric classes (GH-18111)

[bpo-39416](): Document string representations of the Numeric classes

This is a change to the specification of the Python language.

The idea here is to put sane minimal limits on the Python language's default
representations of its Numeric classes.  That way "Marty's Robotic Massage Parlor
and Python Interpreter" implementation of Python won't do anything too
crazy.

Some discussion in the email thread:
Subject: Documenting Python's float.__str__()
https://mail.python.org/archives/list/python-dev@python.org/thread/FV22TKT3S2Q3P7PNN6MCXI6IX3HRRNAL/