Include feature-gated items in bootstrap tool docs

This sets `--all-features` when documenting bootstrap tool crates, and enables
rustdoc's `#![feature(doc_cfg)]` to display which items are feature-gated.

Author: Zalathar

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

@@ -1171,6 +1171,11 @@ fn document_tool(
         }
     }
 
+    // Tell rustdoc to document which items require feature flags.
+    cargo.arg("--all-features");
+    cargo.allow_features("doc_cfg");
+    cargo.rustdocflag("-Zcrate-attr=feature(doc_cfg)");
+
     cargo.rustdocflag("--document-private-items");
     // Since we always pass --document-private-items, there's no need to warn about linking to private items.
     cargo.rustdocflag("-Arustdoc::private-intra-doc-links");