syscall: document Time{spec,val} methods
syscall: document Time{spec,val} methods

Add godoc comments for Time{spec,val} methods Unix and Nano.

Change-Id: I285bbd236af588b30140db7182b05f8b202b5b0b
Reviewed-on: https://go-review.googlesource.com/73271
Run-TryBot: Tobias Klauser <0a68dd4915066ec5d3f81f75a828fee53dcc8822@gmail.com>
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

golang/go,cloudflare/go,CAFxX/go,christopher-henderson/Go,golang/go,akutz/go,josharian/go,CAFxX/go,akutz/go,josharian/go,Cofyc/go,CAFxX/go,nawawi/go,Cofyc/go,josharian/go,josharian/go,cloudflare/go,nawawi/go,CAFxX/go,CAFxX/go,akutz/go,codestation/go,nawawi/go,josharian/go,cloudflare/go,nawawi/go,codestation/go,golang/go,nawawi/go,cloudflare/go,codestation/go,CAFxX/go,codestation/go,akutz/go,akutz/go,codestation/go,CAFxX/go,codestation/go,cloudflare/go,Cofyc/go,cloudflare/go,Cofyc/go,josharian/go,nawawi/go,golang/go,Cofyc/go,golang/go,josharian/go,cloudflare/go,codestation/go,akutz/go,akutz/go,golang/go,akutz/go,josharian/go,Cofyc/go,CAFxX/go,codestation/go,Cofyc/go,golang/go,golang/go,nawawi/go,nawawi/go,cloudflare/go,Cofyc/go