unicode/utf8: document the handling of runes out of range in EncodeRune
    
    Document the way EncodeRune currently handles runes which are
    out of range.  Also add an example showing that behaviour.
    
    Change-Id: I0f8e7645ae053474ec319085a2bb6d7f73bc137c
    Reviewed-on: https://go-review.googlesource.com/c/go/+/255998
    Reviewed-by: Rob Pike <r@golang.org>
    Reviewed-by: Giovanni Bajo <rasky@develer.com>
    Trust: Giovanni Bajo <rasky@develer.com>
    Run-TryBot: Giovanni Bajo <rasky@develer.com>
    TryBot-Result: Go Bot <gobot@golang.org>