Cleanup: move doc-strings into headers, remove duplicates

In some cases move implementation details into the function body.

source/blender/windowmanager/WM_api.h

@@ -1664,7 +1664,16 @@ char WM_event_utf8_to_ascii(const struct wmEvent *event) ATTR_NONNULL(1) ATTR_WA
  */
 bool WM_cursor_test_motion_and_update(const int mval[2]) ATTR_NONNULL(1) ATTR_WARN_UNUSED_RESULT;
 
+/**
+ * Return true if this event type is a candidate for being flagged as consecutive.
+ *
+ * See: #WM_EVENT_IS_CONSECUTIVE doc-string.
+ */
 bool WM_event_consecutive_gesture_test(const wmEvent *event);
+/**
+ * Return true if this event should break the chain of consecutive gestures.
+ * Practically all intentional user input should, key presses or button clicks.
+ */
 bool WM_event_consecutive_gesture_test_break(const wmWindow *win, const wmEvent *event);
 
 int WM_event_drag_threshold(const struct wmEvent *event);