Document overridden roles in Mixy

[JJ]

The problem

Some roles implemented in Baggy do not make sense in Mixy and are then overridden, bailing out with an error. For instance, pick

Suggestions

Translate that to the documentation to make it quite clear and avoid people using it and being surprised

[zoffixznet]

There's a draft in a branch c32b4b35b2 currently blocked by inconsistencies in how we fail and potential lack of spec for those failures: rakudo/rakudo#2109