[issue19697] Document the possible values for __main__.__spec__

[Nick Coghlan]

Nick Coghlan added the comment:

"is some cases" is a typo, but otherwise, yeah, may as well commit this so the website gets updated while we decide what else (if anything) we want to cover.

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue19697>
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