xfrm: add documentation for XFRM_MSG_MIGRATE_STATE

Add documentation for the new XFRM_MSG_MIGRATE_STATE netlink message,
which migrates a single SA identified by SPI and mark without involving
policies.

The document covers the motivation and design differences from the
existing XFRM_MSG_MIGRATE, the SA lookup mechanism, supported attributes
with their omit-to-inherit semantics, and usage examples.

Signed-off-by: Antony Antony <antony.antony@secunet.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>