PDME14 — allow Documentable to point to a DocumentRevision or a DocumentMaster.

This seems to be primarily a ChangeManagement issue. The problem is tracking
the relationship between an ECR and a requirements document. In some cases, it
may be determined that a given change requirement is not quite right for all
product (instances) in a line, so the scope of the first ECR is reduced, the
requirement spec is "revised" and a second ECR is created with the revised
spec. That is, there are two active ECRs with different versions of the "same"
requirements document. (And the PDM Enablers should not require the company's
business process to make that a new DocumentMaster!)

see below