Unfortunately I'm not super-familiar with developments around Polonius, so chances are what I can point you towards are the same things you found when searching. The most relevant bits appear to be the Polonius book [0] linked from the repo [1], but I don't know how up to date the book is or if there are more up-to-date resources. The RFC book [2] doesn't seem to have anything obviously about Polonius either.

[0]: https://rust-lang.github.io/polonius/

[1]: https://github.com/rust-lang/polonius

[2]: https://rust-lang.github.io/rfcs/