clarify that macros can't be called before they're defined

Author: Carl Masak

S06-routines.pod

@@ -9,8 +9,8 @@ Synopsis 6: Subroutines
 
     Created: 21 Mar 2003
 
-    Last Modified: 19 Oct 2014
-    Version: 163
+    Last Modified: 12 Dec 2014
+    Version: 164
 
 This document summarizes Apocalypse 6, which covers subroutines and the
 new type system.
@@ -3142,6 +3142,8 @@ evaluated again, so the two calls below print the same thing:
     say f();    # compile-time call to &f
     say &f();   # runtime call to &f
 
+A compile-time call to a macro before its definition is erroneous.
+
 =head2 Quasiquoting
 
 In aid of returning syntax tree, Perl provides a "quasiquoting"