Add a comment why extern crate is necessary for rustdoc

Author: jyn514

src/librustdoc/lib.rs

@@ -16,6 +16,12 @@
 
 #[macro_use]
 extern crate lazy_static;
+#[macro_use]
+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.
 extern crate rustc_ast;
 extern crate rustc_ast_pretty;
 extern crate rustc_attr;
@@ -42,8 +48,6 @@ extern crate rustc_target;
 extern crate rustc_trait_selection;
 extern crate rustc_typeck;
 extern crate test as testing;
-#[macro_use]
-extern crate tracing;
 
 use std::default::Default;
 use std::env;