Rollup merge of #45308 - dbrgn:headlines, r=steveklabnik
Rollup merge of #45308 - dbrgn:headlines, r=steveklabnik

Rustdoc book: Add missing headlines

Some options were missing headlines, which looked a bit inconsistent.

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