go/types, types2: document nil scope for imported and instantiated Func objects

go/types, types2: document nil scope for imported and instantiated Func objects

Also, don't set the scope anymore when instantiating (substituting)
a signature.

Per discussion with rfindley.

Change-Id: I560d4571c7ff14b0df3e15fece634cb5f9f94a99
Reviewed-on: https://go-review.googlesource.com/c/go/+/363435
Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>