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

* src/parse-gram.y (variable): In comments here.
(cherry picked from commit 904ea1bca411d4d5f4f729e7f2982841a858f5ea)

Conflicts:

	src/parse-gram.c

src/parse-gram.y

@@ -533,6 +533,8 @@ 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); }