go spec: clarify rules for append, scope rules for :=
go spec: clarify rules for append, scope rules for :=

Fixes issue 1936.
Fixes issue 1940.

R=rsc, r, iant, ken, iant
CC=golang-dev
http://codereview.appspot.com/4585049

sanjosh/sanjos100-tipc,sanjosh/sanjos100-tipc,scirelli/scirelli-go,npe9/noahevans-go9,rflanagan/reginaldflanagan-project1,scirelli/scirelli-go,bryanxu/go-zh,d0f/go-zh,scirelli/scirelli-go,Triskite/willstone-goclone,rdp/rogerpack2005-golang,rdp/rogerpack2005-golang,sanjosh/sanjos100-tipc,npe9/noahevans-go9,glycerine/jeaten-go-arrayof-structof,sanjosh/sanjos100-tipc,glycerine/jeaten-go-arrayof-structof,npe9/noahevans-go9,webfd/go-zh,sanjosh/sanjos100-tipc,bryanxu/go-zh,mhennings/marcohennings-go,rflanagan/reginaldflanagan-project1,Triskite/willstone-goclone,scirelli/scirelli-go,rdp/rogerpack2005-golang,mhennings/marcohennings-go,npe9/noahevans-go9,Triskite/willstone-goclone,npe9/noahevans-go9,glycerine/jeaten-go-arrayof-structof,glycerine/jeaten-go-arrayof-structof,npe9/noahevans-go9,scirelli/scirelli-go,mhennings/marcohennings-go,d0f/go-zh,mhennings/marcohennings-go,glycerine/jeaten-go-arrayof-structof,bryanxu/go-zh,webfd/go-zh,glycerine/jeaten-go-arrayof-structof,rdp/rogerpack2005-golang,mhennings/marcohennings-go,glycerine/jeaten-go-arrayof-structof,mhennings/marcohennings-go,d0f/go-zh,mhennings/marcohennings-go,Triskite/willstone-goclone,npe9/noahevans-go9,npe9/noahevans-go9,d0f/go-zh,rflanagan/reginaldflanagan-project1,scirelli/scirelli-go,bryanxu/go-zh,webfd/go-zh,sanjosh/sanjos100-tipc,bryanxu/go-zh,mhennings/marcohennings-go,rflanagan/reginaldflanagan-project1,bryanxu/go-zh,webfd/go-zh,Triskite/willstone-goclone,rdp/rogerpack2005-golang,bryanxu/go-zh,Triskite/willstone-goclone,d0f/go-zh,rdp/rogerpack2005-golang,rflanagan/reginaldflanagan-project1,webfd/go-zh,scirelli/scirelli-go,d0f/go-zh,rdp/rogerpack2005-golang,sanjosh/sanjos100-tipc,scirelli/scirelli-go,d0f/go-zh,d0f/go-zh,Triskite/willstone-goclone,rflanagan/reginaldflanagan-project1,webfd/go-zh,webfd/go-zh,webfd/go-zh,rdp/rogerpack2005-golang,glycerine/jeaten-go-arrayof-structof,sanjosh/sanjos100-tipc,bryanxu/go-zh,Triskite/willstone-goclone,rflanagan/reginaldflanagan-project1,rflanagan/reginaldflanagan-project1