[gtk+] app: improve doc of gtk_application_get_window_by_id()

[Sébastien Wilmet <swilmet src gnome org>]

commit 399e8db3365cebf7b8bd02de7a940203a3b8ed15
Author: Sébastien Wilmet <swilmet gnome org>
Date:   Sun Apr 10 12:56:39 2016 +0200

    app: improve doc of gtk_application_get_window_by_id()
    
    https://bugzilla.gnome.org/show_bug.cgi?id=764846

 gtk/gtkapplication.c |    3 +++
 1 files changed, 3 insertions(+), 0 deletions(-)
---
diff --git a/gtk/gtkapplication.c b/gtk/gtkapplication.c
index df6795a..48a4a89 100644
--- a/gtk/gtkapplication.c
+++ b/gtk/gtkapplication.c
@@ -1159,6 +1159,9 @@ gtk_application_get_windows (GtkApplication *application)
  *
  * Returns the #GtkApplicationWindow with the given ID.
  *
+ * The ID of a #GtkApplicationWindow can be retrieved with
+ * gtk_application_window_get_id().
+ *
  * Returns: (nullable) (transfer none): the window with ID @id, or
  *   %NULL if there is no window with this ID
  *