perf list: Update documentation about raw event setup

It was missing that only certain bit fields are passed to the config
value which confused users. Updating it.

Signed-off-by: Robert Richter <robert.richter@amd.com>
Cc: Ingo Molnar <mingo@kernel.org>
Link: http://lkml.kernel.org/r/1344361396-7237-6-git-send-email-robert.richter@amd.com


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