Let C be whether c valid
Let E be whether err is non-nil

The old comment explicitly says that (~C → E). However, that does call
into question whether (E → ~C), which causes doubts for users.
Without a comment at all, it is obvious that only (E ↔ ~C) makes sense.

Fixes #11308

Change-Id: I5a7d51ceb509057eccca91f57a7e48c9d1c6d112
Reviewed-on: https://go-review.googlesource.com/15256Reviewed-by: Andrew Gerrand <adg@golang.org>