trpl: fix macro follow sets

src/libsyntax/ext/tt/macro_rules.rs

@@ -972,6 +972,7 @@ fn can_be_followed_by_any(frag: &str) -> bool {
 /// we expanded `expr` to include a new binary operator, we might
 /// break macros that were relying on that binary operator as a
 /// separator.
+// when changing this do not forget to update doc/book/macros.md!
 fn is_in_follow(_: &ExtCtxt, tok: &Token, frag: &str) -> Result<bool, String> {
     if let &CloseDelim(_) = tok {
         // closing a token tree can never be matched by any fragment;