perf list: Improve the raw hw event descriptor documentation

perf list: Improve the raw hw event descriptor documentation

It was x86 specific and imcomplete at that, improve the situation by
making it clear where the example provided applies and by adding the
URLs for the Intel and AMD manuals where this is discussed in depth.

Acked-by: Robert Richter <robert.richter@amd.com>
Cc: Cyrill Gorcunov <gorcunov@gmail.com>
Cc: Frédéric Weisbecker <fweisbec@gmail.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Tom Zanussi <tzanussi@gmail.com>
Cc: Robert Richter <robert.richter@amd.com>
Reported-by: Robert Richter <robert.richter@amd.com
LKML-Reference: <new-submission>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>