perf annotate: Add comment about annotated_src->nr_histograms

When we have multiple groups in an evlist, say:

  $ perf stat -e '{cycles,instructions},{cache-references,cache-misses}' sleep 1

   Performance counter stats for 'sleep 1':

           343,134      cycles:u
           249,292      instructions:u            #    0.73  insn per cycle
            15,556      cache-references:u
             8,925      cache-misses:u            #   57.373 % of all cache refs

       1.000957550 seconds time elapsed

  $

Then the perf_evsel instances for the two group leaders ("cycles" and
"cache-references") will have evsel->nr_members set to 2, while all the
evsel->evlist->nr_entries will be set to 4, so we can't use
evsel->evlist->nr_entries everywhere, as event groups need to be taken
into account.

But this probably requires us to audit at least the forced-group code,
where we want all of the events to be in a "group", to see them all in
the screen, one column for each, even knowing that they were not
necessarily scheduled to count at the same time by the kernel perf
subsystem.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Wang Nan <wangnan0@huawei.com>
Link: https://lkml.kernel.org/n/tip-2g0vwqnc49wl4ttjk8dvpgcc@git.kernel.orgSigned-off-by: NArnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/util/annotate.h

@@ -202,6 +202,10 @@ struct cyc_hist {
 /** struct annotated_source - symbols with hits have this attached as in sannotation
  *
  * @histograms: Array of addr hit histograms per event being monitored
+ * nr_histograms: This may not be the same as evsel->evlist->nr_entries if
+ * 		  we have more than a group in a evlist, where we will want
+ * 		  to see each group separately, that is why symbol__annotate2()
+ * 		  sets src->nr_histograms to evsel->nr_members.
  * @lines: If 'print_lines' is specified, per source code line percentages
  * @source: source parsed from a disassembler like objdump -dS
  * @cyc_hist: Average cycles per basic block