lockstat: documentation update

Christoph noted that the documentation doesn't tell in what unit the
lockstat times are reported, ammend this.
Reported-by: NChristoph Hellwig <hch@infradead.org>
Signed-off-by: NPeter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

Documentation/lockstat.txt

@@ -100,6 +100,7 @@ The first lock (05-10) is a read/write lock, and shows two lines above the
 short separator. The contention points don't match the column descriptors,
 they have two: contentions and [<IP>] symbol.
 
+The integer part of the time values is in us.
 
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