Attribute documentation for cfg_attr

[kantnero]

Documents the cfg_attr
Part of #157604

r? @GuillaumeGomez
cc @fmease @traviscross

[rustbot]

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

[fmease]

Meta note:

Instead of trying to add docs for more and more attributes at once in one PR, I think it would be more helpful & generally better to split your efforts into several smaller PRs going forward which each contain a small set of related attributes / just one attribute instead of a large mixed bag.

That would make it a lot easier for reviewers since they wouldn't have to wade through 100 comments like in #158664 and context-switch constantly within the same PR. Consequently, it would mean your PRs can get merged a lot faster.

Re. you closing #158664 I guess that was due to git / GitHub issues? In case it wasn't though, I want to say that it makes it harder to follow the progress & discussions for specific attributes since they're now spread across several PRs (one has 100 comments as I've already mentioned), esp. since there's no link from the old to the new PR.

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In any case, thanks for working on this!

[kantnero]

@fmease @GuillaumeGomez Change this to document just one attr cfg_attr will send subsequent pr after this is merged.