[gtkmm] Added a comment

[Murray Cumming <murrayc src gnome org>]

commit 27c2dd008739d04f4acc421d4c3e8ad1f4bd3c5a
Author: Murray Cumming <murrayc murrayc com>
Date:   Fri Jan 31 09:49:15 2014 +0100

    Added a comment

 gtk/src/popover.hg |    3 +++
 1 files changed, 3 insertions(+), 0 deletions(-)
---
diff --git a/gtk/src/popover.hg b/gtk/src/popover.hg
index 50a706b..aa49993 100644
--- a/gtk/src/popover.hg
+++ b/gtk/src/popover.hg
@@ -67,7 +67,10 @@ public:
 
 #m4 _CONVERSION(`Cairo::RectangleInt&',`cairo_rectangle_int_t*', 
`reinterpret_cast<cairo_rectangle_int_t*>(&$3)')
 #m4 _CONVERSION(`const Cairo::RectangleInt&',`cairo_rectangle_int_t*', `const_cast<cairo_rectangle_int_t* 
> (reinterpret_cast<const cairo_rectangle_int_t* >(&$3))')
+
+  //This cannot take NULL to mean unset.
   _WRAP_METHOD(void set_pointing_to(const Cairo::RectangleInt& rect), gtk_popover_set_pointing_to)
+
   _WRAP_METHOD(bool get_pointing_to(Cairo::RectangleInt& rect) const, gtk_popover_get_pointing_to)
 
   _WRAP_METHOD(void set_position(PositionType position = POS_TOP), gtk_popover_set_position)