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 <mingo@elte.hu>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Robert Richter <robert.richter@amd.com>
LKML-Reference: <1287107460-12112-1-git-send-email-sonnyrao@linux.vnet.ibm.com>
Signed-off-by: Sonny Rao <sonnyrao@linux.vnet.ibm.com>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>