clarify a few things

compiler/rustc_driver_impl/src/lib.rs

@@ -393,6 +393,7 @@ fn run_compiler(
             let early_exit = || early_exit().map(|_| None);
 
             // Parse the crate root source code (doesn't parse submodules yet)
+            // Everything else is parsed during macro expansion.
             queries.parse()?;
 
             // If pretty printing is requested: Figure out the representation, print it and exit