Add a notice about overtesting non-standard syntaxes

[ybiquitous]

Which issue, if any, is this issue related to?

See the discussion on PR #6556.

Is there anything in the PR that needs further explanation?

This PR is just a document fix.

My English wording may be a little unnatural. Any advice is welcome.

[changeset-bot]

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Latest commit: 26a546e

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[ollie-iterators]

Some advice on the wording:

1. Move the second paragraph above the first.
2. "Therefore, a rule can delegate tests for such syntaxes to those utilities in most cases." could be changed to "Therefore, tests for those syntaxes could be delegated to those utilities in most cases"
3. "leads to unhealthy in our project and will be difficult to maintain" could be changed to : "makes it harder to maintain the project"
4. "Please take care of increasing unnecessary tests" could be changed to "Please do not add unnecessary tests"

[jeddy3]

Let's hold off merging this until #6573 lands as it'll be easier to manage the conflict here than there.

[ybiquitous]

@ollie-iterators Thanks for the advice. I've changed it via 26a546e.

@jeddy3 Thanks for writing the docs. If you don't mind, could you merge this PR change into PR #6573 and close this PR? (I'm not stuck on the current wording so that you could arrange it)

[jeddy3]

If you don't mind, could you merge this PR change into PR #6573 and close this PR? (I'm not stuck on the current wording so that you could arrange it)

Sure thing. Done in d7ef445

[ybiquitous]

Thank you!