Add godoc comments for Time{spec,val} methods Unix and Nano.
Also move them to timestruct.go to the other Time{spec,val} related
functionality.

Change-Id: I3b18c5d1bfb235ea4fea25a18fc34b85c21bb365
Reviewed-on: https://go-review.googlesource.com/73871
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>