doc: clarify restrictions to run as non-root

Since modern kernels don't allow unprivileged processes to read the
pagemaps file, the instructions on running as non-root are out-of-date. Add
a note clarifying that they will only work with earlier kernel versions.

Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>
Acked-by: John McNamara <john.mcnamara@intel.com>