Add auto_cfg and no_auto_cfg attributes

[GuillaumeGomez]

Needed for #100883.

I cherry-picked commits from #100883, added documentation and added a warning (and its test) in case feature(doc_auto_cfg) is used without doc(auto_cfg).

Once this PR is merged, we'll wait for 1 release cycle to go back to #100883.

cc @Nemo157
r? @notriddle

[rustbot]

Some changes occurred in src/librustdoc/clean/types.rs

cc @camelid

rustc_error_messages was changed

cc @davidtwco, @compiler-errors, @JohnTitor, @estebank, @TaKO8Ki

[GuillaumeGomez]

Fixed the error.

[GuillaumeGomez]

Updated and rebased. Thanks for the improvements!

[notriddle]

@bors r+

[bors]

📌 Commit 20724ef has been approved by notriddle

It is now in the queue for this repository.

[notriddle]

Actually, no. I want to make sure #100883 (comment) is resolved first.

@bors r-

[bors]

☔ The latest upstream changes (presumably #102948) made this pull request unmergeable. Please resolve the merge conflicts.

[Noratrieb]

Hi, I've seen you changed some diagnostic structs in your PR. After #103345, the way we refer to fluent messages changed. They are now in a flat namespace with the same identifier as in the fluent file. For example, parser::cool_thing is now parser_cool_thing and parser::suggestion just suggestion.
You should rebase to the latest master and change your fluent message references as described above. Thanks!

[GuillaumeGomez]

Forgot to mention but we are writing an RFC for this and for doc(cfg) more generally.

[jyn514]

There are quite a lot of merge conflicts here. I'm going to close this for now until the RFC is accepted.