cmd/doc: add test for constructor, fix build

cmd/doc: add test for constructor, fix build

Most important: skip test on darwin/arm64 for unclear reasons.

First cut at the test missed this feature of go doc: when asking for
the docs for a type, include any function that looks like it constructs
a that type as a return value.

Change-Id: I124e7695e5d365e2b12524b541a9a4e6e0300fbc
Reviewed-on: https://go-review.googlesource.com/11295
Reviewed-by: Rob Pike <r@golang.org>