go/doc: tune factory method association logic

Ignore predeclared types (such as error) in result parameter lists when determining
with which result type a method should be associated with. This change will again
associate common factory functions with the first result type even if there are more
than one result, as long as the others are predeclared types.

Fixes #27928

Change-Id: Ia2aeaed15fc4c8debdeeaf729cc7fbba1612cafb
Reviewed-on: https://go-review.googlesource.com/c/141617


Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>