`rustc_macros` is not in rustc API docs

[camelid]

Maybe because it's a proc-macro crate? It's not on the crate index list, nor is it in the search index, and https://doc.rust-lang.org/nightly/nightly-rustc/rustc_macros doesn't exist.

[ehuss]

Proc-macro documentation is stored in a different directory when --target is used. There is a little more discussion of this at rust-lang/cargo#7677. It is not clear how feasible that is to fix.

[jyn514]

Bootstrap can work around this the same way docs.rs does, by moving compiler-doc/x86_64-unknown-linux-gnu/rustc_macros to compiler-doc/rustc_macros manually.