Add document and comments on marker_synchronize_unregister(): it
should be called before freeing resources that the probes depend on.

Based on comments from Lai Jiangshan and Mathieu Desnoyers.
Signed-off-by: NWu Fengguang <wfg@linux.intel.com>
Reviewed-by: NMathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
Reviewed-by: NLai Jiangshan <laijs@cn.fujitsu.com>
Signed-off-by: NIngo Molnar <mingo@elte.hu>