perf intel-pt: Update documentation about context switch events

perf intel-pt: Update documentation about context switch events

Since the unprivileged sched switch event was added in perf, PT doesn't
need need perf_event_paranoid=-1 anymore for per cpu decoding.

Add a note stating that that is only needed for kernels < 4.2.

Reported-by: Andi Kleen <ak@linux.intel.com>
Link: http://lkml.kernel.org/r/http://lkml.kernel.org/n/tip-x2ybghpqxxn3zu0m8o7qi42r@git.kernel.org
Acked-by: Adrian Hunter <adrian.hunter@intel.com>
Fixes: 45ac1403f564 ("perf: Add PERF_RECORD_SWITCH to indicate context switches")
Link: http://lkml.kernel.org/n/tip-x2ybghpqxxn3zu0m8o7qi42r@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>