Add examples for await-outside-async and fix activation of message

[DudeNr33]

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Type of Changes

	Type
✓	🐛 Bug fix
	✨ New feature
	🔨 Refactoring
✓	📜 Docs

Description

This adds examples and link to the relevant PEP for await-outside-async.
The new test case for checking the message docs actually found a bug (which cost me more time to find than I'd be willing to admit 😀 ):
Because the relevant visit_await method was implemented in the wrong checker class (there are two in the module), it was not possible to disable all messages and then just enable await-outside-async again.
As both the impact and the fix of this are so minor I decided to directly fix this in this PR and only add a ChangeLog entry, without separate issue or whatsnew entry.
Let me know if you'd rather want to split this into two PRs.

Ref: #5953

[DanielNoord]

Thanks @DudeNr33!

Nice to see how this process is finding some hard to spot issue as well. I have also found some missing check_messages through this process 😄

[coveralls]

Pull Request Test Coverage Report for Build 2052735367

• 12 of 12 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 94.154%

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Totals
Change from base Build 2051790443:	0.0%
Covered Lines:	15334
Relevant Lines:	16286

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💛 - Coveralls

[Pierre-Sassoulas]

Because the relevant visit_await method was implemented in the wrong checker class (there are two in the module), it was not possible to disable all messages and then just enable await-outside-async again.

Sounds like something that is supposed to take some time to find out 😄 Thanks for the debug !