perf tools: Update topdown documentation to permit rdpmc calls

Update Topdown documentation to permit calls to rdpmc, and describe
interaction with system calls.

Signed-off-by: Ray Kinsella <mdr@ashroe.eu>
Reviewed-by: Andi Kleen <ak@linux.intel.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Kan Liang <kan.liang@linux.intel.com>
Link: http://lore.kernel.org/lkml/20210421091009.1711565-1-mdr@ashroe.eu


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