[docs] provide the authorative source for s[i:j] negative slice indices (<-len(s)) behavior for standard sequences (issue 29352)

[shadowranger+python at gmail.com]

https://bugs.python.org/review/29352/diff/19815/Doc/library/stdtypes.rst
File Doc/library/stdtypes.rst (right):

https://bugs.python.org/review/29352/diff/19815/Doc/library/stdtypes.rst#newcode937
Doc/library/stdtypes.rst:937: ``len(s)``. If ``len(s) + i`` or ``len(s)
+ j`` is negative, use ``0``. If *i*
I think, to avoid repetition, a footnote cross-reference might make more
sense here:

"If a negative index remains negative after substitution (Note 3), use
``0``."

Or if footnote cross-references are verboten, to match the phrasing for
the previous sentence and simplify the language:

"If *i* or *j* is less than ``-len(s)``, use ``0``."

https://bugs.python.org/review/29352/