builtin: add linkable-to declarations for iota and nil Aids godoc.
builtin: add linkable-to declarations for iota and nil
Aids godoc.

R=golang-dev, gri, bradfitz
CC=golang-dev
https://codereview.appspot.com/7992044

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang