perf_counter: documentation update

Update the documentation to reflect the current state of affairs

[ Impact: documentation update ]
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Corey Ashford <cjashfor@linux.vnet.ibm.com>
LKML-Reference: <20090501102533.296727903@chello.nl>
Signed-off-by: Ingo Molnar <mingo@elte.hu>