buffer: add kernel-doc for block_dirty_folio()

Turn the excellent documentation for this function into kernel-doc. 
Replace 'page' with 'folio' and make a few other minor updates.

Link: https://lkml.kernel.org/r/20240416031754.4076917-3-willy@infradead.org


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