[docs] Clarify that list.insert / list.pop works with negative indexes

[Attila-Mihaly Balazs]

Both list.insert and list.pop support negative indexes and they behave just
like list[negative index]. IMHO this should be called out explicitly in the
documentation - ie:
https://docs.python.org/3.7/tutorial/datastructures.html#

Something like: "a.insert(-1, x) inserts before the last element"

Regards,
Attila
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