go/doc: classify function returning slice or array of T as constructor

go/doc: classify function returning slice or array of T as constructor

Previously, go/doc would only consider functions and slices that
return types of T or any number of pointers to T: *T, **T, etc. This
change expands the definition of a constructor to include functions
that return arrays of a type (or pointer to that type) in its first
return.

With this change, the following return types also classify a function
as a constructor of type T:

[1]T
[1]*T
[1]**T
(and so on)

Fixes #22856.

Change-Id: I37957c5f2d6a7b2ceeb3fbaef359057f2039393d
Reviewed-on: https://go-review.googlesource.com/85355
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>