encoding/json: document that marshaling invalid utf-8 sequence will return error Also added docs for InvalidUTF8Error. Fixes issue 5360.
encoding/json: document that marshaling invalid utf-8 sequence will return error
Also added docs for InvalidUTF8Error.
Fixes issue 5360.

R=golang-dev, adg, r
CC=golang-dev
https://codereview.appspot.com/8926046

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