doc: emphasize that environment variables are optional
doc: emphasize that environment variables are optional

Also update $GOARM description.

R=golang-dev, gri
CC=golang-dev
http://codereview.appspot.com/4972041

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