[release-branch.go1.9] doc: document Go 1.9.3
    
    Change-Id: Ic7a5d3118754b34ab0652fcef889259a03baebc3
    Reviewed-on: https://go-review.googlesource.com/88536
    Reviewed-by: Ian Lance Taylor <iant@golang.org>
    Reviewed-on: https://go-review.googlesource.com/88678
    Run-TryBot: Andrew Bonventre <andybons@golang.org>
    TryBot-Result: Gobot Gobot <gobot@golang.org>
    Reviewed-by: Andrew Bonventre <andybons@golang.org>