Rollup merge of #106383 - Manishearth:ast-docs, r=compiler-errors

Document some of the AST nodes

Someone was confused about some of this on Zulip, added some docs

We probably should make sure every last field/variant in the AST/HIR is documented at some point

`@bors` rollup

diff --cc compiler/rustc_ast/src/ast.rs
Simple merge