perf probe: Clarify x86 register naming for perf probe

Clarify how to specify x86 registers in perf probe. I recently ran into
this problem and had to figure it out from the source.

Signed-off-by: Andi Kleen <ak@linux.intel.com>
Acked-by: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
Cc: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
Link: http://lkml.kernel.org/r/1393596135-4227-3-git-send-email-andi@firstfloor.org


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