cmd/dist: document why test fails on incomplete ports

cmd/dist: document why test fails on incomplete ports

It might not be obvious from reading the code why we consider the test
as failed on incomplete ports even though it passed. Add a comment
documenting this behavior, as suggested by Dmitri in CL 155839.

Change-Id: I3eb7db27d01d63db277172381e5fa51577dad941
Reviewed-on: https://go-review.googlesource.com/c/go/+/264682
Trust: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>