doc/go1.4.txt: flags can now be set at most once
doc/go1.4.txt: flags can now be set at most once

CC=golang-codereviews
https://codereview.appspot.com/161820043

sanjosh/sanjos100-tipc,bryanxu/go-zh,bryanxu/go-zh,bryanxu/go-zh,sanjosh/sanjos100-tipc,webfd/go-zh,webfd/go-zh,webfd/go-zh,d0f/go-zh,d0f/go-zh,d0f/go-zh,bryanxu/go-zh,webfd/go-zh,sanjosh/sanjos100-tipc,sanjosh/sanjos100-tipc,webfd/go-zh,d0f/go-zh,d0f/go-zh,sanjosh/sanjos100-tipc,bryanxu/go-zh,webfd/go-zh,d0f/go-zh,bryanxu/go-zh,sanjosh/sanjos100-tipc,webfd/go-zh,bryanxu/go-zh,d0f/go-zh,d0f/go-zh,sanjosh/sanjos100-tipc,sanjosh/sanjos100-tipc,bryanxu/go-zh,webfd/go-zh