perf diff: Documentation -c cycles option

Documentation the new computation selection 'cycles'.

 v4:
 ---
 Change the column 'Block cycles diff [start:end]' to
 '[Program Block Range] Cycles Diff'
Signed-off-by: NJin Yao <yao.jin@linux.intel.com>
Reviewed-by: NJiri Olsa <jolsa@kernel.org>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Jin Yao <yao.jin@intel.com>
Cc: Kan Liang <kan.liang@linux.intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/1561713784-30533-8-git-send-email-yao.jin@linux.intel.comSigned-off-by: NArnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/Documentation/perf-diff.txt

@@ -90,9 +90,10 @@ OPTIONS
 
 -c::
 --compute::
-        Differential computation selection - delta, ratio, wdiff, delta-abs
-        (default is delta-abs).  Default can be changed using diff.compute
-        config option.  See COMPARISON METHODS section for more info.
+        Differential computation selection - delta, ratio, wdiff, cycles,
+        delta-abs (default is delta-abs).  Default can be changed using
+        diff.compute config option.  See COMPARISON METHODS section for
+        more info.
 
 -p::
 --period::
@@ -280,6 +281,16 @@ If specified the 'Weighted diff' column is displayed with value 'd' computed as:
     - WEIGHT-A being the weight of the data file
     - WEIGHT-B being the weight of the baseline data file
 
+cycles
+~~~~~~
+If specified the '[Program Block Range] Cycles Diff' column is displayed.
+It displays the cycles difference of same program basic block amongst
+two perf.data. The program basic block is the code between two branches.
+
+'[Program Block Range]' indicates the range of a program basic block.
+Source line is reported if it can be found otherwise uses symbol+offset
+instead.
+
 SEE ALSO
 --------
 linkperf:perf-record[1], linkperf:perf-report[1]