[libhandy] swipe-tracker: Add docs for shift_position()

[Adrien Plazas <aplazas src gnome org>]

commit 15b19caf498b9f5b90c4056a11fbf782a6d9ed30
Author: Alexander Mikhaylenko <alexm gnome org>
Date:   Sat Jun 27 00:39:48 2020 +0500

    swipe-tracker: Add docs for shift_position()
    
    This function will be public, it needs docs.
    
    Signed-off-by: Alexander Mikhaylenko <alexm gnome org>

 src/hdy-swipe-tracker.c | 10 ++++++++++
 1 file changed, 10 insertions(+)
---
diff --git a/src/hdy-swipe-tracker.c b/src/hdy-swipe-tracker.c
index f28365dd..6c099b26 100644
--- a/src/hdy-swipe-tracker.c
+++ b/src/hdy-swipe-tracker.c
@@ -1003,6 +1003,16 @@ hdy_swipe_tracker_set_allow_mouse_drag (HdySwipeTracker *self,
   g_object_notify_by_pspec (G_OBJECT (self), props[PROP_ALLOW_MOUSE_DRAG]);
 }
 
+/**
+ * hdy_swipe_tracker_shift_position:
+ * @self: a #HdySwipeTracker
+ * @delta: the position delta
+ *
+ * Move the current progress value by @delta. This can be used to adjust the
+ * current position if snap points move during the gesture.
+ *
+ * Since: 1.0
+ */
 void
 hdy_swipe_tracker_shift_position (HdySwipeTracker *self,
                                   gdouble          delta)