Clarify record reflective support specification

A number of issues and/or concerns have been raised recently relating
to the reflective support for records. These arose when finalizing and
completing the runtime CSR. Taken together they seem to lead back to a
few small, but significant, omissions in the spec that would be good to
tighten up. It is important that it be possible to reflectively reason
about record classes in a way that is unambiguous and provides
certainty that the record class is well-formed.

For a class to be a record class, then:

  1) It's direct superclass must be java.lang.Record, and
  2) It must have a Record attribute.

The entry point to the reflective API for records is the two methods:
Class::isRecord and Class::getRecordComponents.