Remove documentation for std_detect error_macros

[adamgemmell]

These comments show up when there's proper documentation for the runtime feature detection macros, such as here: https://doc.rust-lang.org/nightly/std/macro.is_aarch64_feature_detected.html

The messages also make no sense from an API user's perspective.

[rust-highfive]

r? @Amanieu

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[Amanieu]

I think the proper solution here would be to use cfg(doc) to show the proper documentation for all arch-specific macros when rustdoc is used.

[adamgemmell]

That does seem useful, thanks, I'll update the PR. To me it still makes sense to remove this error documentation - do you agree?

[Amanieu]

Yes, it's fine to change these to normal comments instead of doc comments.

[bors]

☔ The latest upstream changes (presumably f4513d5) made this pull request unmergeable. Please resolve the merge conflicts.

[Amanieu]

Superseded by #1276.