doc/go1.18: add docs on new reflect.Value methods

From:

https://go-review.googlesource.com/c/go/+/352131/

Updates #47694

Change-Id: I7c7811c49900049f5ef7fc906fe25d2ffd73c7b9
Reviewed-on: https://go-review.googlesource.com/c/go/+/368363


Trust: Jeremy Faller <jeremy@golang.org>
Run-TryBot: Jeremy Faller <jeremy@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Fabio Falzoi <fabio.falzoi84@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>