[gtk+] ToolItem: Annotate menu proxy item as nullable

[Daniel Boles <dboles src gnome org>]

commit 7ed0d9344e3bf1e3c5193af22606077509840ea7
Author: Daniel Boles <dboles src gnome org>
Date:   Sun Sep 3 10:39:58 2017 +0100

    ToolItem: Annotate menu proxy item as nullable
    
    It can be NULL. ::create-menu-proxy gave an example of when a NULL proxy
    menu item could be useful, but the accessors did not annotate it as such.

 gtk/gtktoolitem.c |    4 ++--
 1 files changed, 2 insertions(+), 2 deletions(-)
---
diff --git a/gtk/gtktoolitem.c b/gtk/gtktoolitem.c
index a705b97..974ad58 100644
--- a/gtk/gtktoolitem.c
+++ b/gtk/gtktoolitem.c
@@ -849,7 +849,7 @@ gtk_tool_item_retrieve_proxy_menu_item (GtkToolItem *tool_item)
  * @menu_item_ids must match ensures that a #GtkToolItem
  * will not inadvertently change a menu item that they did not create.
  *
- * Returns: (transfer none): The #GtkMenuItem passed to
+ * Returns: (transfer none) (nullable): The #GtkMenuItem passed to
  *     gtk_tool_item_set_proxy_menu_item(), if the @menu_item_ids
  *     match.
  *
@@ -899,7 +899,7 @@ gtk_tool_item_rebuild_menu (GtkToolItem *tool_item)
 
 /**
  * gtk_tool_item_set_proxy_menu_item:
- * @tool_item: a #GtkToolItem
+ * @tool_item: (nullable): a #GtkToolItem
  * @menu_item_id: a string used to identify @menu_item
  * @menu_item: a #GtkMenuItem to be used in the overflow menu
  *