Search backend: add comments to HunkMatch and MatchedContent
Created by: camdencheek
This adds a bunch of comments to HunkMatch since I think I'm happy with where this design landed. Additionally, it adds MatchedContent, which returns the exact set of strings matched by the query. It's not used except in tests yet, but I wanted to add that now to pin that behavior since that's critical to the design of what we store in HunkMatch.
Stacked on #36176
Test plan
Added a test for the one new method. The rest is comments.