Clarify documentation.
Handle unknown multi-parameter attributes.

If mass assignment is used for an unknown single parameter attribute, an
UnknownAttribute error is raised. This makes the behaviour consistent for
multi-parameter attributes.
Raise helpful execptions on unexpected values.

When the attribute does exist, but we don't know what class to use for a
multi-parameter assignment, we now raise a helpful exception.

This should help people TDD their way to a correct implementation of
multi-parameter attributes, or make it more clear what's going on when there's a
real error.