Auto merge of #84283 - jsha:de-emphasize-attributes, r=GuillaumeGomez

rustdoc: Reduce visual weight of attributes.

Followup from #83337. As part of that PR, we stopped hiding attributes behind a toggle, because most things have just zero or one attributes. However, this made clear that the current rendering of attributes emphasizes them a lot, which distracts from function signatures. This PR changes their color of attributes to be the same as the toggles, and reduces their font weight.

This also removes `#[lang]` from the list of ALLOWED_ATTRIBUTES. This attribute is an implementation detail rather than part of the public-facing documentation.

![image](https://user-images.githubusercontent.com/220205/115131061-cc407d80-9fa9-11eb-9a77-ad3f3217f391.png)

Demo at https://hoffman-andrews.com/rust/de-emph-attr/std/string/struct.String.html#method.trim