netdev_driver_t::recv: Added drop and size to doc

netdev_driver_t::recv does on of three things:
 - Receive the frame
 - Drop it
 - Get (an estimation of) the size of the frame

The brief section of the documentation was changed to reflect this. (And it
starts with the least obvious thing in the hope readers of the API stumble upon
it...)

drivers/include/net/netdev.h

@@ -310,7 +310,8 @@ typedef struct netdev_driver {
     int (*send)(netdev_t *dev, const iolist_t *iolist);
 
     /**
-     * @brief Get a received frame
+     * @brief Drop a received frame, **OR** get the length of a received frame,
+     *        **OR** get a received frame.
      *
      * @pre `(dev != NULL)`
      *