perf tools: Document event modifiers

Existing documentation doesn't discuss event modifiers, so add a description of
what's currently possible to the documentation of perf-list.

Cc: Ingo Molnar <[email protected]>
Cc: Paul Mackerras <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Robert Richter <[email protected]>
LKML-Reference: <[email protected]>
Signed-off-by: Sonny Rao <[email protected]>
Signed-off-by: Arnaldo Carvalho de Melo <[email protected]>
1 file changed