[docs] [issue11352] Update cgi module doc

[Senthil Kumaran]

Senthil Kumaran <senthil at uthcode.com> added the comment:

I have made the suggested specific changes, but while reviewing the entire doc, I realized that much improvements can be made. Keeping the documentation more helpful aligned with the cgi.py APIs. I shall keep bug report to update the cgi docs further.

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue11352>
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