lockstat: Add usage info to Documentation/lockstat.txt

Add usage info to Documentation/lockstat.txt

Signed-off-by: Frank Rowand <frank.rowand@am.sony.com>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <4B05BE7D.30500@am.sony.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>