Clarify motivation for E713 and E714 (#11483)
The wording 'negative comparison' is a rather vague description of the
'is not' operation and does not describe what the 'not in' operation
does (potentially copied from 'is not'). This was replaced with more
precise language to describe the operators taken from the official
python docs[1].
Both rules didn't have a strong reasoning besides 'it's bad, use the
other'. The origin of these rules seems to be PEP8[2] which prefers 'is
not' over 'not ... is' for readability. This is now reflected in the
description.
[1]:
https://docs.python.org/3/reference/expressions.html#membership-test-operations
[2]: https://peps.python.org/pep-0008/#programming-recommendations
