docs: ethtool: Add two link extended substates of bad signal integrity

Add documentation for two bad signal integrity substates:
ETHTOOL_LINK_EXT_SUBSTATE_BSI_SERDES_REFERENCE_CLOCK_LOST
ETHTOOL_LINK_EXT_SUBSTATE_BSI_SERDES_ALOS.

Signed-off-by: Guangbin Huang <huangguangbin2@huawei.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>