doc: flesh out Ports section for Go 1.10 release notes

[bradfitz]

https://tip.golang.org/doc/go1.10#ports should provide updates on which OS versions are supported.

• FreeBSD 10.3+ (no more FreeBSD 9.3, as pre-announced in the Go 1.9 release notes)
• NetBSD 8+ (which is working again; the Go 1.9 release notes said NetBSD was broken)
• OpenBSD 6.0+ still, but this will be the last release supporting 6.0. Next release will be 6.2+.

/cc @bsiegert @paulzhol @4a6f656c @mikioh