doc/go1.14: add remarks about range inference and check removal
    
    Mentions CLs 174704 and 196784.
    
    Change-Id: Ia8f821a3d90a4e08c895a6f091dbf07311e885ba
    Reviewed-on: https://go-review.googlesource.com/c/go/+/214946
    Run-TryBot: David Chase <drchase@google.com>
    TryBot-Result: Gobot Gobot <gobot@golang.org>
    Reviewed-by: Austin Clements <austin@google.com>