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

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