The rules are subtle, but under some circumstances the result
can be constant. Mention this and refer to the appropriate
section of the specification.

Fixes #27588.

Change-Id: I4beaad036db87501378fb2ef48d216742d096933
Reviewed-on: https://go-review.googlesource.com/135519Reviewed-by: Robert Griesemer <gri@golang.org>