Clarify how the quote macro is loaded

src/libproc_macro/lib.rs

@@ -236,6 +236,8 @@ pub mod token_stream {
 ///
 /// Unquoting is done with `$`, and works by taking the single next ident as the unquoted term.
 /// To quote `$` itself, use `$$`.
+///
+/// This is a dummy macro, the actual implementation is in quote::Quoter
 #[unstable(feature = "proc_macro", issue = "38356")]
 #[macro_export]
 macro_rules! quote { () => {} }

src/libproc_macro/quote.rs

@@ -21,6 +21,9 @@ use syntax::parse::token;
 use syntax::symbol::Symbol;
 use syntax::tokenstream;
 
+/// This is the actual quote!() proc macro
+///
+/// It is manually loaded in CStore::load_macro_untracked
 pub struct Quoter;
 
 pub fn unquote<T: Into<TokenStream> + Clone>(tokens: &T) -> TokenStream {