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>