doc/go1.16: document os package changes
doc/go1.16: document os package changes

For #39444
For #40700
Fixes #42908

Change-Id: Idae35adecd79e9d7d207f9d78cb009a980e5c8a9
Reviewed-on: https://go-review.googlesource.com/c/go/+/274477
Trust: Tobias Klauser <0a68dd4915066ec5d3f81f75a828fee53dcc8822@gmail.com>
Reviewed-by: Russ Cox <5ad239cb8a44f659eaaee0aa1ea5b94947abe557@golang.org>

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