Rollup merge of #90795 - GuillaumeGomez:more-search-index-comments, r=notriddle
Rollup merge of #90795 - GuillaumeGomez:more-search-index-comments, r=notriddle

Add more comments to explain the code to generate the search index

Fixes #90766.

I tried to put comments when the code wasn't easy to understand at first sight and added more documentation on the recursive function. Please tell me if I misused the terminology or if comments can be improved or added into other places.

r? `@notriddle`

graydon/rust,graydon/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust