emit a warning when polymorphic_on is specfied for a mapper with inherits

[Mike Bayer]

No description provided.

[Mike Bayer]

you can specify polymorphic_on on any mapper. It means that when a query is made against that mapper, that's the column that will be used to determine results. Its normal use case is if the mapper is on a "polymorphic" callable like a union or whatever. If polymorphic_on is None, it gets set against the superclass mapper's polymorphic_on anyway - so if the user has a plain polymorphic setup and sets the attribute correctly on a subclass, i.e. to the column on the root table, there's no difference in configuration.

[Mike Bayer]

Removing milestone: 0.6.5 (automated comment)