Improve docs related to readonly and set readonly to improve clarity of their behaviour

[Firstyear]

Fixes #74895

[rustbot]

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If this PR contains changes to any rust-lang/rust public library APIs then please comment with @rustbot label +T-libs-api -T-libs to tag it appropriately. If this PR contains changes to any unstable APIs please edit the PR description to add a link to the relevant API Change Proposal or create one if you haven't already. If you're unsure where your change falls no worries, just leave it as is and the reviewer will take a look and make a decision to forward on if necessary.

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• Stabilizing library features
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• Changing observable runtime behavior of library APIs

[rust-highfive]

r? @m-ou-se

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[Timmmm]

See also #101599 - I wrote it like a year ago, then forgot to upload it, finally did it today and then saw this!

[JohnTitor]

See also #101599 - I wrote it like a year ago, then forgot to upload it, finally did it today and then saw this!

Did you mean #101644?

[Timmmm]

Ah oops, yes!

[Firstyear]

Closed in favour of #101644

[thomcc]

@Firstyear Did you mean to close this with that comment?

[Firstyear]

Yes, sorry!