[atk] docs: improve documentation for AtkObject::children-changed signal

[Alejandro Piñeiro Iglesias <apinheiro src gnome org>]

commit e4fe9a9b7bb4e314667d4c67f583f6e17232d04e
Author: Alejandro Piñeiro <apinheiro igalia com>
Date:   Wed Sep 11 13:19:15 2013 +0200

    docs: improve documentation for AtkObject::children-changed signal
    
    https://bugzilla.gnome.org/show_bug.cgi?id=684667

 atk/atkobject.c |    4 +++-
 1 files changed, 3 insertions(+), 1 deletions(-)
---
diff --git a/atk/atkobject.c b/atk/atkobject.c
index e809e48..ab8f53e 100755
--- a/atk/atkobject.c
+++ b/atk/atkobject.c
@@ -578,7 +578,9 @@ atk_object_class_init (AtkObjectClass *klass)
   /**
    * AtkObject::children-changed:
    * @atkobject: the object which received the signal.
-   * @arg1: The index of the added or removed child
+   * @arg1: The index of the added or removed child. The value can be
+   * -1. This is used if the value is not known by the implementor
+   * when the child is added or irrelevant.
    * @arg2: A gpointer to the child AtkObject which was added or removed
    *
    * The signal "children-changed" is emitted when a child is added or