Fully remove `notrust` markers from rustdoc

Internally refactor all mentions of `notrust` to the
positive statement `rust`.
auto merge of #20075 : skade/rust/remove-notrust, r=alexcrichton

Internally refactor all mentions of `notrust` to the
positive statement `rust`.

[breaking-change] Change remaining `notrust` markers to
the thing they actually represent, e.g. `sh` or `plain`.

This supersedes #19693 .