Fix up docs for backend setup functions

docs/reference/gio/gio-sections.txt

@@ -2089,6 +2089,10 @@ g_settings_backend_path_writable_changed
 g_settings_backend_writable_changed
 g_settings_backend_changed_tree
 
+<SUBSECTION Custom>
+g_settings_backend_setup
+g_settings_backend_setup_keyfile
+
 <SUBSECTION Standard>
 G_IS_SETTINGS_BACKEND
 G_IS_SETTINGS_BACKEND_CLASS

gio/gkeyfilesettingsbackend.c

@@ -547,6 +547,8 @@ g_keyfile_settings_backend_new (const gchar *filename)
  * not writable, any #GSettings objects created using @context will
  * return %FALSE for any calls to g_settings_is_writable() and any
  * attempts to write will fail.
+ *
+ * Since: 2.26
  */
 void
 g_settings_backend_setup_keyfile (const gchar *context,

gio/gsettingsbackend.c

@@ -906,6 +906,8 @@ g_settings_backend_supports_context (const gchar *context)
  * It is not possible to specify a backend for the default context.
  *
  * This function takes a reference on @backend and never releases it.
+ *
+ * Since: 2.26
  **/
 void
 g_settings_backend_setup (const gchar      *context,