serial: Update documentation regarding RX buffer handling

Relax the expected behavior of the RX path. So far it was stated
that UART_RX_BUF_RELEASED event is generated when one buffer is
completely filled. Now, it is relaxed so that buffer can be
released even when not completed (e.g. after the first timeout).

This approach does not violate the API and may be more convenient
for some devices where on timeout a switch between DMA buffers
must occur.

Signed-off-by: Krzysztof Chruściński <krzysztof.chruscinski@nordicsemi.no>