cmd/doc: incorrect suppression of functions returning unexported types

[dsnet]

Using go1.7rc2

a33e9cf caused a regression from Go1.6.

Consider the following package snippet:

type private int
func Public() private { return 0 }

Currently I see:

$ go doc

I expect to see:

$ go doc
func Public() private

The issue is that the logic in the above commit classified Public as a constructor and hid it under private, thus displaying nothing. Instead, it should always display functions like this.