Rustdoc should show Unstable notices next to re-exported items

[RDambrosio016]

See https://doc.rust-lang.org/std/prelude/v1/index.html
asm is unstable, however there is no unstable tag showing that, it would be nice if it would, just so it can be seen at a glance without going to their respective pages.

cc @jyn514

[jyn514]

Note that rustdoc already does something similar for doc(cfg):
It could use a similar format for re-exports.

[RDambrosio016]

Really could just copy the existing Experimental tags (see https://doc.rust-lang.org/std/index.html -> asm) and put em after the path.

[torhovland]

@rustbot claim