perf tools: Document --children option in more detail

As the --children option changes the output of perf report (and perf
top) it sometimes confuses users. Add more words and examples to help
understanding of the option's behavior - and how to disable it ;-).

Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Reviewed-by: Ingo Molnar <mingo@kernel.org>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Taeung Song <treeze.taeung@gmail.com>
Link: http://lkml.kernel.org/r/1429684425-14987-1-git-send-email-namhyung@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>