buffer: add kernel-doc for brelse() and __brelse()

Move the documentation for __brelse() to brelse(), format it as kernel-doc
and update it from talking about pages to folios.

Link: https://lkml.kernel.org/r/20240416031754.4076917-6-willy@infradead.org


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