[docs] [issue9451] Strengthen __*__ system name warning

[Terry J. Reedy]

Terry J. Reedy <tjreedy at udel.edu> added the comment:

Suggested rewrite:
"System-defined names. These names are defined by the interpreter and its implementation (including the standard library). Current system names are discussed in the Special method names section and elsewhere. More will likely be defined in future versions of Python. *Any* use of __*__ names, in any* context, that does not follow explicitly documented use, is
subject to breakage without warning.

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