madvise.2: Clarify addr/length and update hugetlb support

madvise.2: Clarify addr/length and update hugetlb support

Clarify that madvise only works on full pages, and remove references
to 'bytes'.

Update MADV_DONTNEED and MADV_REMOVE sections to remove notes that
HugeTLB mappings are not supported.  Indicate the releases when they
were first supported as well as alignment restrictions.

Signed-off-by: Mike Kravetz <mike.kravetz@oracle.com>
Acked-by: Peter Xu <peterx@redhat.com>
[alx: wsfix]
Signed-off-by: Alejandro Colomar <alx.manpages@gmail.com>