Rollup merge of #141131 - yotamofek:pr/rustdoc/match-ergo, r=fmease

Make some `match`es slightly more ergonomic in `librustdoc`

Bunch of small cleanups I found while working on other stuff, mostly getting rid of superfluous `*`s and `ref [mut]`s in `match`es,
makes the code less sigil/keyword-heavy, and slightly improves readability IMHO.
Also flattens a few nested `match`es.

Author: matthiaskrgr