bdev: Improve lookup_bdev documentation

Add a Context section and rewrite the rest to be clearer.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Link: https://lore.kernel.org/r/20211213171113.3097631-1-willy@infradead.org


Signed-off-by: Jens Axboe <axboe@kernel.dk>