buffer: improve bdev_getblk documentation

Add some more information about the state of the buffer_head returned.

Link: https://lkml.kernel.org/r/20240416031754.4076917-8-willy@infradead.org


Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Pankaj Raghav <p.raghav@samsung.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>