doc/effective_go.html: update slices and maps. Drop the phrase "reference types", which has caused confusion. Add a section about 2D arrays, a common newbie question.
doc/effective_go.html: update slices and maps.
Drop the phrase "reference types", which has caused confusion.
Add a section about 2D arrays, a common newbie question.

R=golang-dev, cespare, adg, rsc
CC=golang-dev
https://codereview.appspot.com/7423051

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