Auto merge of #36847 - alexcrichton:rustc-macro-doc, r=nrc

rustdoc: Fix documenting rustc-macro crates

This commit adds a "hack" to the session to track whether we're a rustdoc
session or not. If we're rustdoc then we skip the expansion to add the
rustc-macro infrastructure.

Closes #36820

diff --cc src/bootstrap/step.rs
Simple merge

diff --cc src/librustc/session/config.rs
Simple merge