crypto/tls: clarify concurrent use of Config
crypto/tls: clarify concurrent use of Config

LGTM=r, agl
R=agl, r
CC=golang-codereviews
https://codereview.appspot.com/77530044

bryanxu/go-zh,sanjosh/sanjos100-tipc,rdp/rogerpack2005-golang,rdp/rogerpack2005-golang,sanjosh/sanjos100-tipc,sanjosh/sanjos100-tipc,sanjosh/sanjos100-tipc,rdp/rogerpack2005-golang,webfd/go-zh,webfd/go-zh,rdp/rogerpack2005-golang,d0f/go-zh,bryanxu/go-zh,d0f/go-zh,webfd/go-zh,sanjosh/sanjos100-tipc,d0f/go-zh,bryanxu/go-zh,sanjosh/sanjos100-tipc,webfd/go-zh,rdp/rogerpack2005-golang,d0f/go-zh,webfd/go-zh,d0f/go-zh,webfd/go-zh,rdp/rogerpack2005-golang,bryanxu/go-zh,webfd/go-zh,rdp/rogerpack2005-golang,bryanxu/go-zh,d0f/go-zh,webfd/go-zh,d0f/go-zh,sanjosh/sanjos100-tipc,bryanxu/go-zh,d0f/go-zh,bryanxu/go-zh,bryanxu/go-zh,rdp/rogerpack2005-golang,sanjosh/sanjos100-tipc