[docs] [issue18326] Mention 'keyword only' for list.sort, improve glossary.

[R. David Murray]

R. David Murray added the comment:

In this expository context, I prefer the issue18326.that.diff version, but the index link to the keyword-only_parameter ref should still be added to it (on "only be passed by keyword").

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue18326>
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