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.comSigned-off-by: NTom Zanussi <tom.zanussi@linux.intel.com>
Tested-by: NMasami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
Reviewed-by: NNamhyung Kim <namhyung@kernel.org>
Signed-off-by: NSteven Rostedt <rostedt@goodmis.org>