clarify "primary table" in inheritance mappings

In #644 we saw that inheritance mappings are somewhat underspecified, opening up the potential for unexpected (and unnecessary)lack of portability between implementations, and that the term "primary table" is left undefined. We should nail this stuff down.

Start by reading issue #644 and the discussion about inheritance mappings and the undefined term "primary table." Review the relevant Jakarta Persistence specification text to identify the portability ambiguity. Done means the terminology and expected mapping behavior are precisely agreed and documented.

Written by the indexing model from the issue text.