gtkshow: Document that gtk_show_uri is deprecated

Due to the deprecation in gtkshow.h, there is proper deprecation warning
for C code. But, for bindings (python in one case), there is no warning,
due to missing "Deprecated:" annotation in gtkshow.c

Fixes https://gitlab.gnome.org/GNOME/gtk/-/issues/3166

gtk/gtkshow.c

@@ -44,6 +44,8 @@
  * Returns: %TRUE on success, %FALSE on error
  *
  * Since: 2.14
+ *
+ * Deprecated: 3.22: Use gtk_show_uri_on_window() instead.
  */
 gboolean
 gtk_show_uri (GdkScreen    *screen,