doc/go1.14: mention go/doc.NewFromFiles and associated new data fields
    
    Updates #23864
    Updates #36878
    
    Change-Id: I6efdaafbe5207c625643f201a5931ad735941365
    Reviewed-on: https://go-review.googlesource.com/c/go/+/217125
    Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>