Updates #16360.

Adds examples for:
+ Chmod
+ Chtimes
+ FileMode

Change-Id: I1b61ee0392fa3774593a7f36aaf0fa1e484c778b
Reviewed-on: https://go-review.googlesource.com/28963
Run-TryBot: Andrew Gerrand <adg@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Andrew Gerrand <adg@golang.org>