trace: Update documentation for mono, mono_raw and boot clock

trace: Update documentation for mono, mono_raw and boot clock

Documentation was missing for mono and mono_raw, add them and also for
the boot clock introduced in this series.

Signed-off-by: Joel Fernandes <joelaf@google.com>
Signed-off-by: John Stultz <john.stultz@linaro.org>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Steven Rostedt <rostedt@goodmis.org>
Cc: Prarit Bhargava <prarit@redhat.com>
Cc: Richard Cochran <richardcochran@gmail.com>
Link: http://lkml.kernel.org/r/1480372524-15181-8-git-send-email-john.stultz@linaro.org
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>