ei: Expose ei_event_ref()

As with eis, expose ei_event_ref() as well, for consistency.

Part-of: <https://gitlab.freedesktop.org/libinput/libei/-/merge_requests/340>

src/libei-event.h

@@ -86,6 +86,3 @@ ei_event_new_for_device(struct ei_device *device);
 
 struct ei *
 ei_event_get_context(struct ei_event *event);
-
-struct ei_event *
-ei_event_ref(struct ei_event *event);

src/libei.h

@@ -1111,13 +1111,24 @@ struct ei *
 ei_seat_get_context(struct ei_seat *seat);
 
 /**
- * Release resources associated with this event. This function always
+ * Increase the refcount of this struct by one. Use ei_event_unref() to decrease
- * returns NULL.
+ * the refcount. This function always returns the same event that the reference
+ * was added to.
  *
- * The caller cannot increase the refcount of an event. Events should be
+ * Events should be considered transient data and not be held longer than
- * considered transient data and not be held longer than required.
+ * required.
- * ei_event_unref() is provided for consistency (as opposed to, say,
+ */
- * ei_event_free()).
+struct ei_event *
+ei_event_ref(struct ei_event *event);
+
+/**
+ * Decrease the refcount of this struct by one. When the refcount reaches
+ * zero, all allocated resources for this struct are released.
+ *
+ * Events should be considered transient data and not be held longer than
+ * required.
+ *
+ * @return always NULL
  */
 struct ei_event *
 ei_event_unref(struct ei_event *event);