cmd/vet: -composites only checks imported types

The check has worked this way for a long time, but it has never been
well documented.

For #25453.

Change-Id: If603e53348ba51f73b2f449b943c6f97f64aa3eb
Reviewed-on: https://go-review.googlesource.com/113755


Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rob Pike <r@golang.org>