Update rustdoc for feature staging changes

[brson]

cc #21248

I haven't even looked at rustdoc but it's support for stability levels is definitely bitrotted. At the least it needs to not display anything related to stability for non-staged-api cases (most cases).

Nominating because I'd like to fix this for 1.0.

[pnkfelix]

Polish issue for 1.0. P-high.