CORBA23 — is_a underspecified

Summary: the description of is_a suffers from the same problem that we fixed for
non_existent in 2.3: it is not clear what the operation should do if
it could not make a determination due to a hard error. This means that
the application does not know whether failure to make a determination
returns false, or whether that will raise an exception (and false is
returned only if the object doesn"t have the expected type).

clarify