[docs] [issue31504] Documentation for return value for string.rindex is missing when search string is empty

[Ashok Bakthavathsalam]

Ashok Bakthavathsalam added the comment:

Look at my suggested changes. It doesn't add noise, IMHO.

All I am saying is that the explicit behaviour needs to be documented. 
I unnecessarily wasted at least 3-4 hours on this "undocumented" behavior.

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue31504>
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