[gtk+] gadget: Update docs

[Matthias Clasen <matthiasc src gnome org>]

commit 083a17acd4865acdc954e76cab5b4e8ee92d70b2
Author: Matthias Clasen <mclasen redhat com>
Date:   Wed Dec 16 17:32:24 2015 -0500

    gadget: Update docs
    
    Make it clear that out_clip must be set.

 gtk/gtkcsscustomgadget.c |    3 +++
 1 files changed, 3 insertions(+), 0 deletions(-)
---
diff --git a/gtk/gtkcsscustomgadget.c b/gtk/gtkcsscustomgadget.c
index d627a19..60a1881 100644
--- a/gtk/gtkcsscustomgadget.c
+++ b/gtk/gtkcsscustomgadget.c
@@ -76,6 +76,9 @@
  * don't have sub-gadgets of child widgets don't need a GtkCssAllocateFunc
  * (although it is still required to call gtk_css_gadget_allocate() on them).
  *
+ * Note that @out_clip *must* be set to meaningful values. If in doubt,
+ * just set it to the allocation.
+ *
  * GtkCssDrawFunc:
  * @gadget: the #GtkCssCustomGadget
  * @cr: the cairo context to draw on