intl: add comments to _, N_, and G_

gcc/ChangeLog:

	* intl.h: Add comments.

gcc/intl.h

@@ -47,14 +47,18 @@ extern const char *fake_ngettext (const char *singular, const char *plural,
 
 #endif
 
+/* Used to immediately translate the argument.  */
 #ifndef _
 # define _(msgid) gettext (msgid)
 #endif
 
+/* Used to mark strings that will be translated later.  */
 #ifndef N_
 # define N_(msgid) msgid
 #endif
 
+/* Like N_, but for GCC diagnostic format strings.  See ABOUT-GCC-NLS for
+   details.  */
 #ifndef G_
 # define G_(gmsgid) gmsgid
 #endif