Add note about RFC6570 type conversions

The spec doesn't address it, but implementations often have
their own rules.

versions/3.0.4.md

@@ -3524,6 +3524,9 @@ Two cases do offer standards-based guidance:
 * [RFC3987 §3.1](https://datatracker.ietf.org/doc/html/rfc3987#section-3.1) provides guidance for converting non-Unicode strings to UTF-8, particularly in the context of URIs (and by extension, the form media types which use the same encoding rules)
 * [RFC6570 §2.3](https://www.rfc-editor.org/rfc/rfc6570#section-2.3) specifies which values, including but not limited to `null`, are considered _undefined_ and therefore treated specially in the expansion process when serializing based on that specification
 
+Implementations of RFC6570 often have their own conventions for converting non-string values, but these are implementation-specific and not defined by the RFC itself.
+This is one reason for the OpenAPI Specification to leave these conversions as implementation-defined:  It allows using RFC6570 implementations regardless of how they choose to perform the conversions.
+
 To control the serialization of numbers, booleans, and `null` (or other values RFC6570 deems to be undefined) more precisely, schemas can be defined as `type: string` and constrained using `pattern`, `enum`, `format`, and other keywords to communicate how applications must pre-convert their data prior to schema validation.
 The resulting strings would not require any further type conversion.