rustc: Fix `make install`

Move a few docblocks from 'ignore' to something that's not rust (e.g. 'text').

Closes #19678
auto merge of #19701 : alexcrichton/rust/fix-install, r=steveklabnik

Move a few docblocks from 'ignore' to something that's not rust (e.g. 'text').

Closes #19678