RDMA/cm: Add a note explaining how the timewait is eventually freed

The way the cm_timewait_info is converted into a work and then freed
is very subtle and surprising, add a note clarifying the lifetime
here.

Link: https://lore.kernel.org/r/20200506074701.9775-7-leon@kernel.org


Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>