Use "discriminating value" consistently

I found the "discriminator value" language confusing, because
`discriminator` is the field name, and in most cases the value
of a field is the value in the OpenAPI Description.

Reviewers did not like "discriminator property value", but
"discriminating value" makes it clear that this is about the
value that actually causes schema selection, and not about
the value of the discriminator keyword.

Also removed placeholder in favor of just having the headings,
as has been done in several other PRs that overlap with
new section PRs for internal linking.