[issue25237] Add doc for tkinter commondialog.Dialog and subclasses

[Nikhil Patel]

Nikhil Patel added the comment:

I've written rst files containing documentation for the tkinter modules and submitted a PR. Attempted to mimic the wording as well as style of existing documentation to keep it consistent.

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nosy: +npatel

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue25237>
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