time: revise Timer comments for Stop, Reset

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/31350


Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>