docs/migration: Split "dirty limit"

Split that into a separate file, put under "features".

Cc: Yong Huang <yong.huang@smartx.com>
Reviewed-by: Cédric Le Goater <clg@redhat.com>
Link: https://lore.kernel.org/r/20240109064628.595453-8-peterx@redhat.com
Signed-off-by: Peter Xu <peterx@redhat.com>
3 files changed