Rollup merge of rust-lang#150022 - GuillaumeGomez:rustc-macro-expansion, r=kobzol

Generate macro expansion for rust compiler crates docs

This enables the `--generate-macro-expansion` rustdoc flag, generating possibility to expand macros directly in source code pages (rust-lang#137229).

Needed this new feature when I was working on rust-lang#149919 and I thought "why not enable it by default?". So here we go.

Not too sure who to r? here so:

r? `@kobzol`

Author: JonathanBrouwer

src/bootstrap/src/core/build_steps/doc.rs

@@ -932,6 +932,7 @@ impl Step for Rustc {
         // see https://github.com/rust-lang/rust/pull/122066#issuecomment-1983049222
         // If there is any bug, please comment out the next line.
         cargo.rustdocflag("--generate-link-to-definition");
+        cargo.rustdocflag("--generate-macro-expansion");
 
         compile::rustc_cargo(builder, &mut cargo, target, &build_compiler, &self.crates);
         cargo.arg("-Zskip-rustdoc-fingerprint");