perf mem: Clarify load-latency in documentation

Clarify in the documentation that 'perf mem report' reports use-latency,
not load/store-latency on Intel systems.

This often causes confusion with users.

Signed-off-by: Andi Kleen <ak@linux.intel.com>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/r/1393596135-4227-2-git-send-email-andi@firstfloor.org


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>