doc/go1.17: document io/fs changes for Go 1.17
doc/go1.17: document io/fs changes for Go 1.17

For #44513
Fixes #46011

Change-Id: I862ef9a4314cd34fb8c828a8cd7d0a7b36c6f683
Reviewed-on: https://go-review.googlesource.com/c/go/+/320151
Reviewed-by: Heschi Kreinick <e89ab2b172855aa0ad01c40c4d8a3535a61ac68a@google.com>
Trust: Heschi Kreinick <e89ab2b172855aa0ad01c40c4d8a3535a61ac68a@google.com>
Trust: Robert Findley <9179eee875d1ee628c37fb47d7e749e855ffd21f@google.com>
Trust: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@golang.org>

CAFxX/go,nawawi/go,cloudflare/go,golang/go,golang/go,nawawi/go,cloudflare/go,nawawi/go,golang/go,nawawi/go,golang/go,nawawi/go,CAFxX/go,cloudflare/go,golang/go,CAFxX/go,cloudflare/go,CAFxX/go,cloudflare/go,golang/go,CAFxX/go,cloudflare/go,nawawi/go,CAFxX/go,golang/go,CAFxX/go,golang/go,cloudflare/go,cloudflare/go,nawawi/go,nawawi/go,CAFxX/go