[gtk+] Add a few missing (nullable) annotations

[Timm Bäder <baedert src gnome org>]

commit 548f6138ef674445c0f19f03db5e9d99f7d43ce5
Author: Timm Bäder <mail baedert org>
Date:   Thu Feb 16 12:22:33 2017 +0100

    Add a few missing (nullable) annotations

 gtk/gtkbutton.c  |    2 +-
 gtk/gtkgesture.c |    4 ++--
 gtk/gtkimage.c   |    2 +-
 3 files changed, 4 insertions(+), 4 deletions(-)
---
diff --git a/gtk/gtkbutton.c b/gtk/gtkbutton.c
index 7ac14e5..df8e455 100644
--- a/gtk/gtkbutton.c
+++ b/gtk/gtkbutton.c
@@ -661,7 +661,7 @@ gtk_button_new_with_label (const gchar *label)
 
 /**
  * gtk_button_new_from_icon_name:
- * @icon_name: an icon name
+ * @icon_name: (nullable): an icon name or %NULL
  * @size: (type int): an icon size (#GtkIconSize)
  *
  * Creates a new button containing an icon from the current icon theme.
diff --git a/gtk/gtkgesture.c b/gtk/gtkgesture.c
index 3d75510..64426c8 100644
--- a/gtk/gtkgesture.c
+++ b/gtk/gtkgesture.c
@@ -1523,12 +1523,12 @@ _gtk_gesture_check (GtkGesture *gesture)
 /**
  * gtk_gesture_handles_sequence:
  * @gesture: a #GtkGesture
- * @sequence: a #GdkEventSequence
+ * @sequence: (nullable): a #GdkEventSequence or %NULL
  *
  * Returns %TRUE if @gesture is currently handling events corresponding to
  * @sequence.
  *
- * Returns: %TRUE if @gesture is handling @sequence
+ * Returns: %TRUE if @gesture is handling @sequence, %FALSE otherwise
  *
  * Since: 3.14
  **/
diff --git a/gtk/gtkimage.c b/gtk/gtkimage.c
index d788270..1ed309f 100644
--- a/gtk/gtkimage.c
+++ b/gtk/gtkimage.c
@@ -1063,7 +1063,7 @@ gtk_image_set_from_gicon  (GtkImage       *image,
 /**
  * gtk_image_set_from_surface:
  * @image: a #GtkImage
- * @surface: a cairo_surface_t
+ * @surface: (nullable): a cairo_surface_t or %NULL
  *
  * See gtk_image_new_from_surface() for details.
  *