Rollup merge of #153004 - Zalathar:tweak-modifiers, r=wesleywiser

Superficial tweaks to the query modifier docs in `rustc_middle::query::modifiers`

This PR sorts the dummy items in the `modifiers` module, makes them non-pub to speed up find-all-references, and adds some extra explanation of what the dummy modifier items are for.

(These dummy items mostly just exist to carry documentation, and have no actual effect on compiler behaviour.)

Author: JonathanBrouwer