doc/go1.16: document crypto/x509 changes for Go 1.16

[dmitshur]

As of filing this issue, the Go 1.16 draft release notes contained the following HTML:

crypto/x509

[...]

TODO: https://golang.org/cl/257257: return additional chains from Verify on Windows

TODO: https://golang.org/cl/262343: add Unwrap to SystemRootsError

---

Per golang.org/s/release, it's a release blocker for Go 1.16 Beta 1 to have a complete draft of the eventual release notes, and so the TODO needs to be resolved.

A sequence of steps to resolve this issue may be:

1. Come up with a draft release note entry in this issue, then move the issue to NeedsFix state.
2. Send a CL with "doc/go1.16:" prefix and include "For doc: write Go 1.16 release notes #40700. Fixes #{this issue}." in the body.

[gopherbot]

Change https://golang.org/cl/274239 mentions this issue: doc/go1.16: add crypto/x509 note about Verify on Windows

[gopherbot]

Change https://golang.org/cl/275313 mentions this issue: doc/go1.16: cleanup crypto release notes