Fix documentation for ECClass modifier

[ColinKerr]

Current docs are:

modifier identifies the class as abstract or sealed.

Valid options are:

None (default) – normal, instantiable class. Not valid for ECRelationshipClass type.
Abstract – abstract class, cannot be instantiated.
Sealed – normal, instantiable class but cannot be used as a base class or have children

At minimum the note that None is not valid on relationships is incorrect. Review and validate other claims