docs/vm: hugetlbpage: move section about kernel development to hugetlbfs_reserv

The hugetlbpage describes hugetlbfs from the user perspective and newer
hugetlbfs_reserv document targets kernel developers. Hence the section
about hugetlbfs kernel development naturally belongs there.

Signed-off-by: Mike Rapoport <rppt@linux.vnet.ibm.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>