doc: Don't imply incorrect guarantees about data races.
doc: Don't imply incorrect guarantees about data races.

A race between
        a = "hello, world"
and
        print(a)
is not guaranteed to print either "hello, world" or "".
Its behaviour is undefined.

Fixes issue 4039.

R=rsc
CC=dvyukov, gobot, golang-dev, r
http://codereview.appspot.com/6489075

webfd/go-zh,glycerine/jeaten-go-arrayof-structof,glycerine/jeaten-go-arrayof-structof,scirelli/scirelli-go,bryanxu/go-zh,rflanagan/reginaldflanagan-project1,rflanagan/reginaldflanagan-project1,glycerine/jeaten-go-arrayof-structof,glycerine/jeaten-go-arrayof-structof,bryanxu/go-zh,glycerine/jeaten-go-arrayof-structof,bryanxu/go-zh,d0f/go-zh,rflanagan/reginaldflanagan-project1,rdp/rogerpack2005-golang,mhennings/marcohennings-go,bryanxu/go-zh,rflanagan/reginaldflanagan-project1,d0f/go-zh,rdp/rogerpack2005-golang,webfd/go-zh,scirelli/scirelli-go,mhennings/marcohennings-go,d0f/go-zh,sanjosh/sanjos100-tipc,sanjosh/sanjos100-tipc,rdp/rogerpack2005-golang,mhennings/marcohennings-go,rflanagan/reginaldflanagan-project1,webfd/go-zh,scirelli/scirelli-go,glycerine/jeaten-go-arrayof-structof,scirelli/scirelli-go,rflanagan/reginaldflanagan-project1,scirelli/scirelli-go,sanjosh/sanjos100-tipc,rdp/rogerpack2005-golang,sanjosh/sanjos100-tipc,mhennings/marcohennings-go,rdp/rogerpack2005-golang,rdp/rogerpack2005-golang,webfd/go-zh,rflanagan/reginaldflanagan-project1,sanjosh/sanjos100-tipc,webfd/go-zh,bryanxu/go-zh,mhennings/marcohennings-go,webfd/go-zh,scirelli/scirelli-go,mhennings/marcohennings-go,d0f/go-zh,d0f/go-zh,bryanxu/go-zh,rdp/rogerpack2005-golang,webfd/go-zh,scirelli/scirelli-go,scirelli/scirelli-go,webfd/go-zh,d0f/go-zh,sanjosh/sanjos100-tipc,sanjosh/sanjos100-tipc,glycerine/jeaten-go-arrayof-structof,bryanxu/go-zh,mhennings/marcohennings-go,sanjosh/sanjos100-tipc,glycerine/jeaten-go-arrayof-structof,rdp/rogerpack2005-golang,bryanxu/go-zh,mhennings/marcohennings-go,d0f/go-zh,rflanagan/reginaldflanagan-project1,d0f/go-zh