Documentation/ABI/rtrs-clt: Add descriptions for min-latency policy

Describe new multipath policy min-latency of the RTRS client.

Link: https://lore.kernel.org/r/20210407113444.150961-5-gi-oh.kim@ionos.com


Signed-off-by: Gioh Kim <gi-oh.kim@ionos.com>
Signed-off-by: Jack Wang <jinpu.wang@ionos.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>