Clarify resolving implicit connections (3.1.1)

This clarifies how to handle resolving implicit (non-URI-based)
connections in multi-document OpenAPI Descriptions.

While the behavior is implementation-defined overall, this
RECOMMENDS a single approach based on how things behaved going
back to the 2.0 referencing model.  This allows Security Schemes
and Tags to (like the top-level Server Objects) define
a deployment-specific interface for referenced documents to access.

This entry document interface approach makes less sense for the
Discriminator Object, but it can use the URI syntax of `mapping`
to keep things within the local document.

This also aligns the search for matching `operationId`s with
3.1's full-document parsing requirements.

Note that the term "complete OpenAPI document" has been defined
in another change pending approval on the 3.0.4 branch.