syscall: drop references to Unix epoch in Timeval/Timespec docs

The various conversion functions just change the format of time values.
They don't use the Unix epoch. Although in practice the values are often
times since the Unix epoch, they aren't always, so referring to the
epoch can be confusing.

Fixes #43010

Change-Id: I640d665f0d2017f0974db05d70858037c7c91eda
Reviewed-on: https://go-review.googlesource.com/c/go/+/277073


Trust: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>