Rollup merge of #65761 - popzxc:document-ast, r=petrochenkov

libsyntax: Enhance documentation of the AST module

This PR enhances documentation state to the `libsyntax/ast.rs` (as initiative caused by [rustc-guide#474](https://github.com/rust-lang/rustc-guide/issues/474)), by adding:
- Module documentation.
- Doc-comments (and a bit of usual comments) in non-obvious (as for me) places.
- Minor style fixes to improve module readability.