perf tool: add example to perf-timechart documentation

While at it, update the synopsis to show both forms.

Cc: Ingo Molnar <mingo@kernel.org>
Cc: Namhyung Kim <namhyung@gmail.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Arnaldo Carvalho de Melo <acme@ghostprotocols.net>