tracing: Add 'hist' trigger Documentation

Add documentation and usage examples for 'hist' triggers.

Link: http://lkml.kernel.org/r/2e13f35f47fea6d647f0efefccfc9673ea84b29f.1457029949.git.tom.zanussi@linux.intel.com

Signed-off-by: Tom Zanussi <[email protected]>
Tested-by: Masami Hiramatsu <[email protected]>
Reviewed-by: Namhyung Kim <[email protected]>
Signed-off-by: Steven Rostedt <[email protected]>

Author: tzanussi