doc/go1.16: document embed, io/fs, runtime/metrics
doc/go1.16: document embed, io/fs, runtime/metrics

Fixes #42915.

Change-Id: Ia6e205aaac3cbf4ba7340deafad444ac3e573559
Reviewed-on: https://go-review.googlesource.com/c/go/+/275114
Trust: Russ Cox <5ad239cb8a44f659eaaee0aa1ea5b94947abe557@golang.org>
Run-TryBot: Russ Cox <5ad239cb8a44f659eaaee0aa1ea5b94947abe557@golang.org>
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

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