Fix MultiIndex last type param default / docs

[maurolacy]

With the latest implementation of type-safe bounds, the definition of a MultiIndex PK type is crucial, as it now serves to define the type of the (partial) bounds over the index key (the part that corresponds to the pk, that is). See https://github.com/confio/poe-contracts/pull/114/files#r812029191 for reference / an example.

Let's fix the docs to reflect this use case, and its importance.

Also, let's remove the default over PK that we introduced for backwards compatibility, as this type definition is now really important for specifying (type-safe) bounds.