[gtk/scroll-compression] gdk: Add documentation

[Matthias Clasen <matthiasc src gnome org>]

commit d605d9ae73507f6b10a6651af8bf86cb8f1c9ac7
Author: Matthias Clasen <mclasen redhat com>
Date:   Tue Jun 9 13:59:26 2020 -0400

    gdk: Add documentation
    
    Add some useful information about event history.

 gdk/gdkevents.c | 6 ++++++
 1 file changed, 6 insertions(+)
---
diff --git a/gdk/gdkevents.c b/gdk/gdkevents.c
index 759f8a0128..e67f5b66cb 100644
--- a/gdk/gdkevents.c
+++ b/gdk/gdkevents.c
@@ -2899,6 +2899,12 @@ gdk_motion_event_new (GdkSurface      *surface,
  *
  * Retrieves the history of the @event, as a list of time and coordinates.
  *
+ * The history includes events that are not delivered to the application
+ * because they occurred in the same frame as @event.
+ *
+ * Note that only motion and scroll events record history, and motion
+ * events only if one of the mouse buttons is down.
+ *
  * Returns: (transfer container) (array length=out_n_coords) (nullable): an
  *   array of time and coordinates
  */