a) Allow multiple AT decls for in a single instance
b) Allow a free type parameter to be instantiated

Example   class C a where
            type T a x :: *

	  data A
          data B
          instance C Int where
            type T Int A = Int
            type T Int B = Bool

There is no reason to prohibit this, and as we move
towards a proper kind system it may even be useful.

I also updated the documentation to cover this change
and the previous one of allowing free type parameters
for associated families.