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

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

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

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

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

Fixes #18063.

Change-Id: I9a1a689933e13c6b8eb80b74ceec85bd4cab236d
Reviewed-on: https://go-review.googlesource.com/54971
Reviewed-by: Robert Griesemer <gri@golang.org>