Make comment in fmgr.h match the one in fmgr.c.

Incompletely quoting an API spec does nobody any good.  Noted by
Paul Jungwirth.  Looks like the discrepancy was my fault originally :-(

Discussion: https://postgr.es/m/CA+renyU_J8TU_d3Kr0PkuOgFbpypextendu7a+_d5NOfVdvDeA@mail.gmail.com

src/include/fmgr.h

@@ -489,7 +489,8 @@ extern int no_such_variable
 
 /* These are for invocation of a specifically named function with a
  * directly-computed parameter list.  Note that neither arguments nor result
- * are allowed to be NULL.
+ * are allowed to be NULL.  Also, the function cannot be one that needs to
+ * look at FmgrInfo, since there won't be any.
  */
 extern Datum DirectFunctionCall1Coll(PGFunction func, Oid collation,
 									 Datum arg1);