doc/go1.17: document reflect changes

For #44513.
Fixes #46019.

Change-Id: Ica84edd5703a4ccf343ff01e10483f8d51d1c79f
Reviewed-on: https://go-review.googlesource.com/c/go/+/323069


Trust: Damien Neil <dneil@google.com>
Run-TryBot: Damien Neil <dneil@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>