Add some missing nullable return annotations

gdk/gdkclipboard.c

@@ -681,7 +681,7 @@ gdk_clipboard_read_async (GdkClipboard        *clipboard,
  *
  * Finishes an asynchronous clipboard read started with gdk_clipboard_read_async().
  *
- * Returns: (transfer full): a #GInputStream or %NULL on error.
+ * Returns: (transfer full) (nullable): a #GInputStream or %NULL on error.
  **/
 GInputStream *
 gdk_clipboard_read_finish (GdkClipboard  *clipboard,

gdk/gdkcontentformats.c

@@ -385,7 +385,7 @@ gdk_content_formats_match_gtype (const GdkContentFormats *first,
  * in @second. If no matching mime type is found, %NULL is
  * returned.
  *
- * Returns: The first common mime type or %NULL if none.
+ * Returns: (nullable): The first common mime type or %NULL if none.
  **/
 const char *
 gdk_content_formats_match_mime_type (const GdkContentFormats *first,

gdk/gdkdisplay.c

@@ -1107,7 +1107,7 @@ gdk_display_notify_startup_complete (GdkDisplay  *display,
  * Gets the startup notification ID for a Wayland display, or %NULL
  * if no ID has been defined.
  *
- * Returns: the startup notification ID for @display, or %NULL
+ * Returns: (nullable): the startup notification ID for @display, or %NULL
  */
 const char *
 gdk_display_get_startup_notification_id (GdkDisplay *display)

gdk/wayland/gdkdisplay-wayland.c

@@ -873,7 +873,7 @@ gdk_wayland_display_get_next_serial (GdkDisplay *display)
  * Gets the startup notification ID for a Wayland display, or %NULL
  * if no ID has been defined.
  *
- * Returns: the startup notification ID for @display, or %NULL
+ * Returns: (nullable): the startup notification ID for @display, or %NULL
  */
 const char *
 gdk_wayland_display_get_startup_notification_id (GdkDisplay  *display)

gsk/gsktransform.c

@@ -1684,7 +1684,7 @@ gsk_transform_transform (GskTransform *next,
  * between those cases, you should check @self is not %NULL
  * before calling this function.
  *
- * Returns: The inverted transform or %NULL if the transform
+ * Returns: (nullable): The inverted transform or %NULL if the transform
  *     cannot be inverted.
  **/
 GskTransform *

gtk/gtkbuilderparser.c

@@ -296,7 +296,7 @@ gtk_buildable_parse_context_pop (GtkBuildableParseContext *context)
  * give the element_name as passed to those functions. For the parent
  * elements, see gtk_buildable_parse_context_get_element_stack().
  *
- * Returns: the name of the currently open element, or %NULL
+ * Returns: (nullable): the name of the currently open element, or %NULL
  */
 const char *
 gtk_buildable_parse_context_get_element (GtkBuildableParseContext *context)

gtk/gtkentrycompletion.c

@@ -1329,7 +1329,7 @@ gtk_entry_completion_real_insert_prefix (GtkEntryCompletion *completion,
  * Get the original text entered by the user that triggered
  * the completion or %NULL if there’s no completion ongoing.
  *
- * Returns: the prefix for the current completion
+ * Returns: (nullable): the prefix for the current completion
  */
 const char *
 gtk_entry_completion_get_completion_prefix (GtkEntryCompletion *completion)

gtk/gtkexpression.c

@@ -1026,7 +1026,7 @@ gtk_object_expression_new (GObject *object)
  *
  * Gets the object that the expression evaluates to.
  *
- * Returns: (transfer none): the object, or %NULL
+ * Returns: (transfer none) (nullable): the object, or %NULL
  */
 GObject *
 gtk_object_expression_get_object (GtkExpression *expression)

gtk/gtkstyleproperty.c

@@ -128,7 +128,7 @@ _gtk_style_property_init (GtkStyleProperty *property)
  * always be a #GtkCssValue whose values can be queried with
  * _gtk_css_array_value_get_nth().
  *
- * Returns: %NULL on failure or the parsed #GtkCssValue
+ * Returns: (nullable): %NULL on failure or the parsed #GtkCssValue
  **/
 GtkCssValue *
 _gtk_style_property_parse_value (GtkStyleProperty *property,

gtk/gtktreemodel.c

@@ -623,7 +623,7 @@ gtk_tree_path_new (void)
  * child of that 11th child, and the 1st child of that 5th child.
  * If an invalid path string is passed in, %NULL is returned.
  *
- * Returns: A newly-created #GtkTreePath-struct, or %NULL
+ * Returns: (nullable): A newly-created #GtkTreePath-struct, or %NULL
  */
 GtkTreePath *
 gtk_tree_path_new_from_string (const char *path)
@@ -866,7 +866,7 @@ gtk_tree_path_get_depth (GtkTreePath *path)
  *
  * The length of the array can be obtained with gtk_tree_path_get_depth().
  *
- * Returns: The current indices, or %NULL
+ * Returns: (nullable): The current indices, or %NULL
  */
 int *
 gtk_tree_path_get_indices (GtkTreePath *path)
@@ -2286,7 +2286,7 @@ gtk_tree_row_reference_unref_path (GtkTreePath  *path,
  * propagated, and the path is updated appropriately. If
  * @path isn’t a valid path in @model, then %NULL is returned.
  *
- * Returns: a newly allocated #GtkTreeRowReference, or %NULL
+ * Returns: (nullable): a newly allocated #GtkTreeRowReference, or %NULL
  */
 GtkTreeRowReference *
 gtk_tree_row_reference_new (GtkTreeModel *model,
@@ -2333,7 +2333,7 @@ gtk_tree_row_reference_new (GtkTreeModel *model,
  * need to carefully monitor exactly when a row reference updates
  * itself, and is not generally needed by most applications.
  *
- * Returns: a newly allocated #GtkTreeRowReference, or %NULL
+ * Returns: (nullable): a newly allocated #GtkTreeRowReference, or %NULL
  */
 GtkTreeRowReference *
 gtk_tree_row_reference_new_proxy (GObject      *proxy,