Clarify that CDI does not use standard APIs to remove a SFSB

By applying a scope to a Stateful Session Bean, CDI gets the responsibility to at some point destroy this SFSB (when the scope ends).

However, destroying a SFSB other then by calling a method annotated with `@Remove` is not something a user can normally do. There is no standard API for this in EJB.

CDI is capable of doing this by calling into implementation specific EJB APIs, but this is not overly clear when reading the spec and could be clarified.