doc: update Effective Go for template API change.
doc: update Effective Go for template API change.

The original change 3378041 was submitted in December 2010:
template: change the signature of formatters for future development.

R=adg
CC=golang-dev
http://codereview.appspot.com/3862041

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