To simplify the pkg syscall docs.

Change-Id: I3ed0bfc59564ada280141c4662a56e141d197e1f
Reviewed-on: https://go-review.googlesource.com/127656
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>