perf report: Add mem-mode documentation to report command

Add mem-mode sorting types and mem-mode itself to perf-report documentation.

Signed-off-by: Don Zickus <dzickus@redhat.com>
Link: http://lkml.kernel.org/r/1400526833-141779-5-git-send-email-dzickus@redhat.com


Signed-off-by: Jiri Olsa <jolsa@kernel.org>