Documentation required for new policy cache feature

For the 5.6.0.0 release, the new policy cache feature will be available as a non-supported technology preview.

There is a section at the end of https://docs.google.com/document/d/1mEUZQ1nYm0wkcOQCbUBa8t4y7LdpE9CszGPyk2n5xZg/edit which contains details about what we think would need to go into the docs.