Document newly introduced modes for the LED netdev trigger.

Add documentation for new modes:
- link_2500
- link_5000
- link_10000
Signed-off-by: Daniel Golle <daniel@makrotopia.org>
Link: https://lore.kernel.org/r/e72a6794639cf3881d698e1d34b456e747da1b95.1701143925.git.daniel@makrotopia.orgSigned-off-by: Lee Jones <lee@kernel.org>