Document methods on Enforcer interface

[retzkek]

When Enforcer was changed to an interface, with methods implemented on the stdEnforcer type, all the method documentation was hidden, also breaking many of the links in the README. The documentation should be moved into the interface definition.

https://pkg.go.dev/github.com/scitokens/scitokens-go#Enforcer
https://github.com/scitokens/scitokens-go/blob/master/enforcer.go#L95