Rollup merge of rust-lang#47732 - mark-i-m:markim_comments_0001, r=jseyfried

Run rustfmt and add doc comments to libsyntax/ext/tt/macro_parser.rs

Similar to rust-lang#47603

cc @theotherphil since you reviewed my other PR 😄

And because they are already assigned on rust-lang#47603:

r? @arielb1

Author: kennytm