docs/migration: Add the dirty limit section

The dirty limit feature has been introduced since the 8.1
QEMU release but has not reflected in the document, add a
section for that.

Signed-off-by: Hyman Huang <yong.huang@smartx.com>
Reviewed-by: Fabiano Rosas <farosas@suse.de>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>
Message-ID: <0f2b2c63fec22ea23e4926cdeb567b7a0ebd8152.1698847223.git.yong.huang@smartx.com>