cmd/doc: ensure functions with unexported return values are shown

The commit in golang.org/cl/22354 groups constructors functions under
the type that they construct to. However, this caused a minor regression
where functions that had unexported return values were not being printed
at all. Thus, we forgo the grouping logic if the type the constructor falls
under is not going to be printed.

Fixes #16568

Change-Id: Idc14f5d03770282a519dc22187646bda676af612
Reviewed-on: https://go-review.googlesource.com/25369


Run-TryBot: Joe Tsai <thebrokentoaster@gmail.com>
Reviewed-by: Rob Pike <r@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>