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

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

Fixes #2820.

R=rsc
CC=golang-dev
https://golang.org/cl/5604046

src/pkg/go/doc/reader.go

@@ -488,6 +488,7 @@ var predeclaredTypes = map[string]bool{
 	"byte":       true,
 	"complex64":  true,
 	"complex128": true,
+	"error":      true,
 	"float32":    true,
 	"float64":    true,
 	"int":        true,
@@ -495,6 +496,7 @@ var predeclaredTypes = map[string]bool{
 	"int16":      true,
 	"int32":      true,
 	"int64":      true,
+	"rune":       true,
 	"string":     true,
 	"uint":       true,
 	"uint8":      true,