Documentation: vm.txt: Adding 'nr_hugepages_mempolicy' parameter description.

This patch adds 'nr_hugepages_mempolicy' parameter which is currently missing
in 'vm.txt' file. It also contains a short description of 'nr_hugepages_mempolicy'
parameter.

Signed-off-by: Prashant Dhamdhere <pdhamdhe@redhat.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>