madvise.2: update THP file/shmem documentation for +5.4

madvise.2: update THP file/shmem documentation for +5.4

Since Linux 5.4, Transparent Huge Pages now support both file-backed
memory and shmem memory. Update MADV_HUGEPAGE advice description to
reflect this.

Additionally, expand the description of requirements for memory to be
considered eligible for THP: alignment / mapping requirements, VMA
flags, prctl(2) settings, inode status, etc.

Signed-off-by: Zach O'Keefe <zokeefe@google.com>
[ alx: ffix ]
Signed-off-by: Alejandro Colomar <alx@kernel.org>