[gi-docgen] docs: Add the `callback` fragment to the list

[Emmanuele Bassi <ebassi src gnome org>]

commit 0503d27ecb5f59f71badc3f6656365c689125f91
Author: Emmanuele Bassi <ebassi gnome org>
Date:   Tue May 25 18:06:55 2021 +0100

    docs: Add the `callback` fragment to the list
    
    We use the `callback` fragment to link to callback types.

 docs/linking.rst | 3 +++
 1 file changed, 3 insertions(+)
---
diff --git a/docs/linking.rst b/docs/linking.rst
index b1f0447..70cf8df 100644
--- a/docs/linking.rst
+++ b/docs/linking.rst
@@ -56,6 +56,9 @@ The available qualifier fragments are:
    * - ``alias``
      - An alias to another type
      - ``[alias@Allocation]``
+   * - ``callback``
+     - A callback type
+     - ``[callback@Gtk.ListBoxForeachFunc]``
    * - ``class``
      - An object class
      - ``[class@Widget]``, ``[class@Gdk.Surface]``, ``[class@Gsk.RenderNode]``