net: phylink: Document MAC_(A)SYM_PAUSE

This documents the possible MLO_PAUSE_* settings which can result from
different combinations of MAC_(A)SYM_PAUSE. Special note is paid to
settings which can result from user configuration (MLO_PAUSE_AN). The
autonegotiation results are more-or-less a direct consequence of IEEE
802.3 Table 28B-2.

Signed-off-by: Sean Anderson <sean.anderson@seco.com>
Signed-off-by: David S. Miller <davem@davemloft.net>