Rollup merge of rust-lang#53931 - iirelu:keyword-docs, r=steveklabnik

Gradually expanding libstd's keyword documentation

I'm working on adding new keywords to the documentation and refreshing the incomplete older ones, and I'm hoping that I can eventually add all the standalone-usable keywords after a bunch of incremental work. It would be cool to see the keywords section of std's docs be a definitive reference as to what each keyword means when you see it, and that's what I'm aiming towards with this work.

I'm far from a Rust expert so there will inevitably be things to fix in this, also I'm not sure if this should be a bunch of quickly-merged PRs or one gradually-updated PR that gets merged once it's done.