Document how `%define "var" "value"' is not M4-friendly.

* src/parse-gram.y (variable): In comments here.

src/parse-gram.y

@@ -545,6 +545,8 @@ rhs:
  | variable and content.opt.  |
  *---------------------------*/
 
+/* The STRING form of variable is deprecated and is not M4-friendly.
+   For example, M4 fails for `%define "[" "value"'.  */
 variable:
   ID
 | STRING { $$ = uniqstr_new ($1); }