tracing/doc: Add latency tracer funcgraph example

This add an example about how to use funcgraph with latency tracers.

Link: http://lkml.kernel.org/r/20190101154614.8887-6-changbin.du@gmail.com



Signed-off-by: Changbin Du <changbin.du@gmail.com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>