perf bench numa: Add more comment for -c option

Adding more commentary for -c/--show_convergence option, to explain how
the convergence is defined.

Before:
    -c, --show_convergence
                          show convergence details

Now:
    -c, --show_convergence
                          convergence is reached when each process \
	(all its threads) is running on a single NUMA node.

Suggested--by: Jiri Hladky <jhladky@redhat.com>
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Hladky <jhladky@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1488732011-27384-1-git-send-email-jolsa@kernel.org


[ Rephrased a bit based on a IRC conversation with Jiri ]
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>