encoding/json: document angle bracket escaping.
encoding/json: document angle bracket escaping.

Fixes issue 2643.

R=rsc, d_smithson
CC=golang-dev
http://codereview.appspot.com/5543068

gnanderson/go,pombredanne/go-deleteme,dsriram/golang,gnanderson/go,dsriram/golang,apparentlymart/golang,tv42/old-go,gnanderson/go,tv42/old-go,dsriram/golang,pombredanne/go-deleteme,dsriram/golang,tv42/old-go,tv42/old-go,dsriram/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,pombredanne/go-deleteme,tv42/old-go,pombredanne/go-deleteme,tv42/old-go,pombredanne/go-deleteme,apparentlymart/golang,apparentlymart/golang,gnanderson/go,pombredanne/go-deleteme,dsriram/golang,gnanderson/go,pombredanne/go-deleteme,gnanderson/go,dsriram/golang,gnanderson/go,apparentlymart/golang,tv42/old-go