[docs] [issue17232] Improve -O docs

[Terry J. Reedy]

Terry J. Reedy added the comment:

I corrected my copy of the .diff.

Since this issue is so far focused on removing the false optimize claim, hiding .pyx info is a new sub-issue. I will follow whatever the consensus is, but since this is a cpython-specific doc and help, I would prefer to give complete info. In fact, I would like to add 'stored in __pycache__' or even 'hidden away in  __pycache__', the latter to suggest that most people should generally forget about them. On Windows, _xxx files like __pycache__ appear in both Command Prompt dir and Explorer file listings, so beginners need to known that __cache__ is both normal and ignorable.

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue17232>
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