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

[changbin . du]

From: Changbin Du <changbin.du@xxxxxxxxx>

Commit 2f3f9bcf000b ("perf tools: Add +field argument support for --field
option") by Jiri Olsa <jolsa@xxxxxxxxxx> 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@xxxxxxxxx>
---
 tools/perf/Documentation/perf-report.txt | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/tools/perf/Documentation/perf-report.txt b/tools/perf/Documentation/perf-report.txt
index c04cc06..c831ebc 100644
--- a/tools/perf/Documentation/perf-report.txt
+++ b/tools/perf/Documentation/perf-report.txt
@@ -172,6 +172,9 @@ OPTIONS
 	By default, every sort keys not specified in -F will be appended
 	automatically.
 
+	If the keys starts with a prefix '+', then will append field(s) to
+	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
-- 
2.7.4