[gtk+] Clarify docs for gdk_rectangle_union

[Matthias Clasen <matthiasc src gnome org>]

commit 51f05a00aef5e54f4ecd006f49ed9d1d0497aae8
Author: Matthias Clasen <mclasen redhat com>
Date:   Wed Jan 20 13:12:13 2016 -0500

    Clarify docs for gdk_rectangle_union
    
    This function does not ignore empty rectangles. Since this
    is a fairly subtle point about the behavior, it is worth
    spelling this out in the documentation. We've had a bug
    open about this for a long time:
    
    https://bugzilla.gnome.org/show_bug.cgi?id=464528

 gdk/gdkrectangle.c |    3 +++
 1 files changed, 3 insertions(+), 0 deletions(-)
---
diff --git a/gdk/gdkrectangle.c b/gdk/gdkrectangle.c
index 84abd2f..491de6c 100644
--- a/gdk/gdkrectangle.c
+++ b/gdk/gdkrectangle.c
@@ -58,6 +58,9 @@
  * The union of rectangles @src1 and @src2 is the smallest rectangle which
  * includes both @src1 and @src2 within it.
  * It is allowed for @dest to be the same as either @src1 or @src2.
+ *
+ * Note that this function does not ignore 'empty' rectangles (ie. with
+ * zero width or height).
  */
 void
 gdk_rectangle_union (const GdkRectangle *src1,