perf report: Document +field style argument support for --field option

Commit 2f3f9bcf ("perf tools: Add +field argument support for
--field option") by Jiri Olsa <jolsa@kernel.org> introduced +field style
argument support for --field option.

This is useful but not updated documentation.  This add a little
description there.
Signed-off-by: Changbin Du <changbin.du@intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/20170313083252.23644-1-changbin.du@intel.com
[ Slightly improved the phrase structure ]
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/Documentation/perf-report.txt

@@ -173,6 +173,9 @@ OPTIONS
 	By default, every sort keys not specified in -F will be appended
 	automatically.
 
+	If the keys starts with a prefix '+', then it will append the specified
+        field(s) to the default field order. For example: perf report -F +period,sample.
+
 -p::
 --parent=<regex>::
         A regex filter to identify parent. The parent is a caller of this