Hm, I had a very different experience. The extensive example library makes the Phaser documentation excellent, in my opinion. The API docs are average, by no means terrible. I've used Phaser, Unity, or SDL for various projects and I've hit the documentation fairly hard for each, so I hope I might have some perspective on this.

http://phaser.io/examples

I've had a completely different experience. Phaser is one of the easiest to use JavaScript game engines and the documentation is well written, also they have plenty of examples. If you prefer to learn from a book, there are also a few books available.