Rollup merge of rust-lang#31639 - quodlibetor:doc-search-with-unknown…

…-types, r=alexcrichton

This enables `*` in all type positions in doc searches, which I often
want in order to find functions that create or convert specific
types (e.g. `* -> vec`) but I don't actually know what kinds of input
they expect.

I actually started working on this because of rust-lang#31598, but I've wanted it
several times when exploring new crates.