Rollup merge of #124091 - jieyouxu:ast-validation-top-level-docs, r=wesleywiser

Update AST validation module docs

Drive-by doc update for AST validation pass:

- Syntax extensions are replaced by proc macros.
- Add rationale for why AST validation pass need to be run
  post-expansion and why the pass is needed in the first place.

This was discussed during this week's [rustc-dev-guide reading club](https://rust-lang.zulipchat.com/#narrow/stream/196385-t-compiler.2Fwg-rustc-dev-guide), and the rationale was explained by cc ``````@bjorn3.``````

Author: matthiaskrgr