doc: Improve calculation of vm.nr_hugepages
doc: Improve calculation of vm.nr_hugepages

The previous method worked off the full virtual address space, not just
the shared memory usage.

Author: Tsunakawa, Takayuki <03b6dee107e557382fdd1ec7eb19a8a04b4cfcf5@jp.fujitsu.com>
Reviewed-by: Justin Pryzby <ca289b655fccb1d52de932b0853f82a8386c7d66@telsasoft.com>
Reviewed-by: Vasundhar Boddapati <d0dcb6d5eddf2c504c06b00d794d7b4466033b20@gmail.com>

adam8157/gpdb,adam8157/gpdb,xinzweb/gpdb,adam8157/gpdb,greenplum-db/gpdb,50wu/gpdb,greenplum-db/gpdb,adam8157/gpdb,50wu/gpdb,xinzweb/gpdb,lisakowen/gpdb,adam8157/gpdb,lisakowen/gpdb,lisakowen/gpdb,greenplum-db/gpdb,adam8157/gpdb,greenplum-db/gpdb,xinzweb/gpdb,50wu/gpdb,xinzweb/gpdb,xinzweb/gpdb,50wu/gpdb,greenplum-db/gpdb,lisakowen/gpdb,lisakowen/gpdb,50wu/gpdb,lisakowen/gpdb,xinzweb/gpdb,adam8157/gpdb,50wu/gpdb,50wu/gpdb,greenplum-db/gpdb,greenplum-db/gpdb,adam8157/gpdb,xinzweb/gpdb,greenplum-db/gpdb,lisakowen/gpdb,lisakowen/gpdb,50wu/gpdb,xinzweb/gpdb