perf tools: Add some comments to the event definitions

Just to make it clear that these are _not_ generic event
structures but do rely on the counter configuration.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Corey J Ashford <cjashfor@us.ibm.com>
Cc: Paul Mackerras <paulus@samba.org>
Cc: stephane eranian <eranian@googlemail.com>
LKML-Reference: <20090813103655.334194326@chello.nl>
Signed-off-by: Ingo Molnar <mingo@elte.hu>