Documentation: networking: add missing PLCA messages from the message list

Physical Layer Collision Avoidance messages are correctly documented but
were left-out of the global list of ethnl messages, add them to the
list.

Signed-off-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
Link: https://lore.kernel.org/r/20231130191400.817948-1-maxime.chevallier@bootlin.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>