It’s a bit hidden, because it’s in the documentation for the parent class, pm.Distribution, and even there there isn’t much in the way of a description. As you may be able to tell, the documentation is in the middle of a major revision in preparation for the release of version 4. But if that page or other aspects of the documentation aren’t what you need, feel free to post questions here and someone should be able to get you headed in the right direction!
[Edit: check out the v3 doc for a bit more info.]
[Edit 2: or check out this docstring.]