[3.1.0-dev] Improve "Discriminator object" section of the OAS specification

[sebastien-rosset]

This is the same PR as #2216. I mistakenly closed #2216 and I cannot find a way to re-open it.

Original description (copied by @MikeRalphson)

This PR attempts to clarify the Discriminator Object section of OAS 3.1. Discriminators seem to cause a number of questions, confusion and interoperability issues. I think some of these questions could be addressed by formulating the normative statements more precisely. I am aware of #2143 but a goal of deprecating the discriminator object is not in conflict with improving the existing specification.