block: bdrv_insert_node(): doc and style

 - options & flags is common pair for open-like functions, let's use it
 - add a comment that specifies use of @options

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <20210920115538.264372-4-vsementsov@virtuozzo.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>