It was only visible when you run godoc with explicit GOOS=windows,
which is less useful for people developing portable application on
non-windows platforms.

Also added a note that log/syslog is not supported on NaCl.

Change-Id: I81650445fb2a5ee161da7e0608c3d3547d5ac2a6
Reviewed-on: https://go-review.googlesource.com/9245Reviewed-by: Ian Lance Taylor <iant@golang.org>