doc/go1.18: add docs on FieldByIndexErr

From:
https://go-review.googlesource.com/c/go/+/357962/

Updates #47694

Change-Id: I2d43dcbd28c03457b55eada26e87f7710a113b0c
Reviewed-on: https://go-review.googlesource.com/c/go/+/368364


Trust: Jeremy Faller <jeremy@golang.org>
Run-TryBot: Jeremy Faller <jeremy@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>