[docs] [issue31942] Document that support of start and stop parameters in the Sequence's index() is optional

[Roundup Robot]

Change by Roundup Robot <devnull at psf.upfronthosting.co.za>:


----------
pull_requests: +4706

_______________________________________
Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue31942>
_______________________________________