doc/install: describe GOHOSTOS and GOHOSTARCH
doc/install: describe GOHOSTOS and GOHOSTARCH

Fixes issue 1303.

R=rsc, PeterGo
CC=golang-dev
http://codereview.appspot.com/3442041

Committer: Andrew Gerrand <395a7d33bec8475c9b83b7d440f141bcbd994aa5@golang.org>

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