perf tool: add example to perf-trace documentation

It's not clear that perf trace must be run as a privileged user, so
include an example demonstrating its usage with sudo. While at it,
update the synopsis to include options.

Cc: Ingo Molnar <mingo@kernel.org>
Cc: Namhyung Kim <namhyung@gmail.com>
Cc: Arnaldo Carvalho de Melo <acme@ghostprotocols.net>
Cc: David Ahern <dsahern@gmail.com>
Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>