Mention GtkIconSize in all docs where the argument/return is annotated as taking an int instead of GtkIconSize

In https://bugzilla.gnome.org/show_bug.cgi?id=601425 the annotations
were changed to int as they not only take the predefined enum values
but also user defined values registered through gtk_icon_size_register()

As a result the typelib doesn't contain any information about
GtkIconSize for those arguments and the Python docstring only
shows the corresponding Python type "int".

This changes the argument docs to mention the type explicitly
so the Python doc generator can add a link to Gtk.IconSize
which contains the most useful predefined values.

https://bugzilla.gnome.org/show_bug.cgi?id=757411

gtk/gtkrender.c

@@ -1097,8 +1097,9 @@ gtk_render_icon_pixbuf_unpacked (GdkPixbuf           *base_pixbuf,
  * gtk_render_icon_pixbuf:
  * @context: a #GtkStyleContext
  * @source: the #GtkIconSource specifying the icon to render
- * @size: (type int): the size to render the icon at. A size of (GtkIconSize) -1
- *        means render at the size of the source and don’t scale.
+ * @size: (type int): the size (#GtkIconSize) to render the icon at.
+ *        A size of `(GtkIconSize) -1` means render at the size of the source
+ *        and don’t scale.
  *
  * Renders the icon specified by @source at the given @size, returning the result
  * in a pixbuf.