New beginners are not familiar with open(2)-style masking of the
flags. Add an example demonstrates the flag or'ing.

Change-Id: Ifa8009c55173ba0dc6642c1d3b3124c766b1ebbb
Reviewed-on: https://go-review.googlesource.com/27996Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>