doc/go1.14: mention new field go/build.Context.Dir
    
    Updates #34860
    Updates #36168
    Updates #36878
    
    Change-Id: I484d7fea5d77d6dcd451d4fdffe0c450eed73636
    Reviewed-on: https://go-review.googlesource.com/c/go/+/217124
    Reviewed-by: Bryan C. Mills <bcmills@google.com>