go/doc: added error, rune to list of predeclared types
go/doc: added error, rune to list of predeclared types

Don't throw away factory functions returning error or rune.

Fixes issue 2820.

R=rsc
CC=golang-dev
http://codereview.appspot.com/5604046

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