rustdoc: don't render the `auto` of auto traits

[fmease]

Uplifted from #t-lang/Removal of auto trait syntax (comment).

We currently render the contextual keyword auto in the docs of auto traits. E.g., for std::marker::Sync, we display the following code block at the top of the page:

pub unsafe auto trait Sync { }

However, the feature auto_traits is no longer planned to be stabilized¹. Instead it will likely be marked as internal soon².
It might even be possible that the syntax auto trait will be removed altogether (see #116126).

Therefore, I don't think rustdoc should display this effectively internal syntax in the public docs. As a replacement for it, it should add a section or similar to the page of auto traits shortly explaining what the special semantics of the given trait are and potentially link to some more elaborate docs.

If this doesn't get claimed within one month, I will work on it.

Footnotes

1. #t-lang/Removal of auto trait syntax (comment) ↩

2. #t-lang/Removal of auto trait syntax (comment) ↩