Add a comment why rustdoc loads crates from the sysroot

Author: jyn514

src/librustdoc/lib.rs

@@ -21,7 +21,11 @@ extern crate tracing;
 
 // N.B. these need `extern crate` even in 2018 edition
 // because they're loaded implicitly from the sysroot.
-// Dependencies listed in Cargo.toml do not need extern crate.
+// The reason they're loaded from the sysroot is because
+// the rustdoc artifacts aren't stored in rustc's cargo target directory.
+// So if `rustc` was specified in Cargo.toml, this would spuriously rebuild crates.
+//
+// Dependencies listed in Cargo.toml do not need `extern crate`.
 extern crate rustc_ast;
 extern crate rustc_ast_pretty;
 extern crate rustc_attr;