It was correct for an early version of the CL which introduced the
type, but later versions of the CL changed the behavior without
updating the documentation.

Fixes #12568

Change-Id: Ia4090a02ba122e9f8317ed86c4c9839ae2c539e0
Reviewed-on: https://go-review.googlesource.com/14496Reviewed-by: Ian Lance Taylor <iant@golang.org>