[glib] Add a small note to g_cclosure_marshal_generic docs

[Paolo Borelli <pborelli src gnome org>]

commit 4a4b0443212c27c5386b1f2ebf35883734915464
Author: Paolo Borelli <pborelli gnome org>
Date:   Thu Apr 3 11:33:33 2014 +0200

    Add a small note to g_cclosure_marshal_generic docs
    
    reviewed-by: Emmanuele Bassi

 gobject/gclosure.c |    3 +++
 1 files changed, 3 insertions(+), 0 deletions(-)
---
diff --git a/gobject/gclosure.c b/gobject/gclosure.c
index 02e9d86..b516b05 100644
--- a/gobject/gclosure.c
+++ b/gobject/gclosure.c
@@ -1369,6 +1369,9 @@ va_to_ffi_type (GType gtype,
  * A generic marshaller function implemented via
  * [libffi](http://sourceware.org/libffi/).
  *
+ * Normally this function is not passed explicitly to g_signal_new(),
+ * but used automatically by GLib when specifying a %NULL marshaller.
+ *
  * Since: 2.30
  */
 void