doc/go1.16: document that on OpenBSD syscalls are now made through libc
doc/go1.16: document that on OpenBSD syscalls are now made through libc

Updates #36435, #40700.

Change-Id: I1e2ded111ad58066cc9f2c9d00e719497b0f34d8
Reviewed-on: https://go-review.googlesource.com/c/go/+/287634
Trust: Cherry Zhang <d62e63aa42ce272d7b6a5055d97e942b33a34679@google.com>
Reviewed-by: Joel Sing <4816dabf8db1bc6cac35b3a24cab2ff844b5b0c7@sing.id.au>

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