block: simplify comment to BDRV_REQ_SERIALISING

block: simplify comment to BDRV_REQ_SERIALISING

1. BDRV_REQ_NO_SERIALISING doesn't exist already, don't mention it.

2. We are going to add one more user of BDRV_REQ_SERIALISING, so
   comment about backup becomes a bit confusing here. The use case in
   backup is documented in block/backup.c, so let's just drop
   duplication here.

3. The fact that BDRV_REQ_SERIALISING is only for write requests is
   omitted. Add a note.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Alberto Garcia <berto@igalia.com>
Message-Id: <20201021145859.11201-2-vsementsov@virtuozzo.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>