[docs] [issue33252] Clarify ResourceWarning documentation

[Ed Morley]

New submission from Ed Morley <emorley at mozilla.com>:

The `ResourceWarning` warning has always been ignored by default, since it was added in Python 3.2 in:
https://github.com/python/cpython/commit/08be72d0aa0112118b79d271479598c218adfd23#diff-db4e2b9efea108a38c53e06fa99cdd77R391

However there are several places in the docs where the "ignored by default" aspect is not mentioned even though it is for `DeprecationWarning` and similar - which caused me quite a bit of confusion today.

The docs were partly improved in Python 3.7 onwards by:
https://github.com/python/cpython/commit/9b99747386b690007027c3be2a5d7cfe3d3634f5

...however:
(a) there is still at least one place that still needs updating on master (https://docs.python.org/3.8/library/warnings.html#warning-categories)
(b) it would be good to backport the docs fixes to 3.4/3.5/3.6 (I was looking at the 3.6 docs today)

I'm happy to open PRs to fix this; but filing this issue so I have something to reference.

----------
assignee: docs at python
components: Documentation
messages: 315153
nosy: docs at python, edmorley
priority: normal
severity: normal
status: open
title: Clarify ResourceWarning documentation
type: enhancement
versions: Python 3.4, Python 3.5, Python 3.6, Python 3.7, Python 3.8

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue33252>
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