More Discoverable Type Augmentation in Docs

[hmaiermack]

Hi, wondering if it might make sense to add type augmentation documentation to the Getting Started page. It seems that a very common use case would be to extend the DefaultSession type to include a userId to be used in DB queries etc. It took me quite a bit of digging to find this as a solution as the callback documentation doesn't reference needing to extend the type alongside implementing a custom callback. Ultimately my fault for not reading the typescript portion of the documentation closely, but seems like something that should be referenced outside of the typescript section of the docs.