madvise.2: add documentation for MADV_COLLAPSE

madvise.2: add documentation for MADV_COLLAPSE

Linux 6.1 introduced MADV_COLLAPSE in upstream commit 7d8faaf15545
("mm/madvise: introduce MADV_COLLAPSE sync hugepage collapse") and
upstream commit 34488399fa08 ("mm/madvise: add file and shmem support to
MADV_COLLAPSE").  Update the man-pages for madvise(2) and
process_madvise(2).

Link: https://lore.kernel.org/linux-mm/20220922224046.1143204-1-zokeefe@google.com/
Link: https://lore.kernel.org/linux-mm/20220706235936.2197195-1-zokeefe@google.com/
Signed-off-by: Zach O'Keefe <zokeefe@google.com>
[ alx: Change use of hyphens in 3-word compound adjective ]
Signed-off-by: Alejandro Colomar <alx@kernel.org>