The comments added for Go 1.7 are very close.
Make explicit that they only apply if the timer is
not known to have expired already.

Fixes #14038.

Change-Id: I6a38be7b2015e1571fc477e18444a8cee38aab29
Reviewed-on: https://go-review.googlesource.com/31350Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>