perf diff: Update perf diff documentation for multiple data comparison

Updating perf diff documentation to include multiple perf data files
comparison.

Signed-off-by: Jiri Olsa <jolsa@redhat.com>
Reviewed-by: Namhyung Kim <namhyung@kernel.org>
Cc: Corey Ashford <cjashfor@linux.vnet.ibm.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/n/tip-tr6su3wfm20k2m5npjggyvtw@git.kernel.org


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>