doc/go1.18: split bytes and strings packages

A small touchup after CL 361894.

For #47694.

Change-Id: Ifc161516f897f727195d21351a3c8eda7b6e327e
Reviewed-on: https://go-review.googlesource.com/c/go/+/361895


Trust: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Carlos Amedee <carlos@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Run-TryBot: Carlos Amedee <carlos@golang.org>