netdev_driver_t::recv: Extend doc on size feature

Some drivers return on netdev_driver_t::recv() not the actual frame size, but
an upper bound estimation of it. The upper layer code is already prepared to
handle that. This brings the doc in sync with the code.

drivers/include/net/netdev.h

@@ -318,8 +318,8 @@ typedef struct netdev_driver {
      * Supposed to be called from
      * @ref netdev_t::event_callback "netdev->event_callback()"
      *
-     * If @p buf == NULL and @p len == 0, returns the packet size without
-     * dropping it.
+     * If @p buf == NULL and @p len == 0, returns the packet size -- or an upper
+     * bound estimation of the size -- without dropping the packet.
      * If @p buf == NULL and @p len > 0, drops the packet and returns the packet
      * size.
      *
@@ -342,7 +342,7 @@ typedef struct netdev_driver {
      *
      * @return `-ENOBUFS` if supplied buffer is too small
      * @return number of bytes read if buf != NULL
-     * @return packet size if buf == NULL
+     * @return packet size (or upper bound estimation) if buf == NULL
      */
     int (*recv)(netdev_t *dev, void *buf, size_t len, void *info);