madvise.2: Document MADV_POPULATE_READ and MADV_POPULATE_WRITE

madvise.2: Document MADV_POPULATE_READ and MADV_POPULATE_WRITE

MADV_POPULATE_READ and MADV_POPULATE_WRITE have been merged into
upstream Linux via commit 4ca9b3859dac ("mm/madvise: introduce
MADV_POPULATE_(READ|WRITE) to prefault page tables"), part of v5.14-rc1.

Further, commit eb2faa513c24 ("mm/madvise: report SIGBUS as -EFAULT for
MADV_POPULATE_(READ|WRITE)"), part of v5.14-rc6, made sure that SIGBUS is
converted to -EFAULT instead of -EINVAL.

Let's document the behavior and error conditions of these new madvise()
options.

Acked-by: Pankaj Gupta <pankaj.gupta@ionos.com>
Cc: Michael Kerrisk <mtk.manpages@gmail.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: Jann Horn <jannh@google.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Linux API <linux-api@vger.kernel.org>
Cc: linux-mm@kvack.org
Signed-off-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Alejandro Colomar <alx.manpages@gmail.com>