Improve wording around binary and serializedValue

Co-authored-by: Dan Hudlow <[email protected]>

Author: handrews

src/oas.md

@@ -2171,7 +2171,7 @@ Note that `serializedValue`, which MUST be a string, is by necessity a sequence
 This conversion is purely one of encoding.  All escaping necessary to reduce the example to the set of characters valid for the target ___location (e.g. URI percent-encoding) MUST be included in the `serializedValue`.
 If the target ___location's encoding is ambiguous, `externalSerializedValue` can be used to demonstrate the exact serialization, including character set encoding, that is intended.
 
-While converting from one character encoding to another is well-defined, binary data does not always correspond to valid Unicode strings.
+`serializedValue` can be used for any textual value with a character set encoding which has an unambiguous mapping to Unicode code points. Since Unicode strings have no inherent binary representation, `serializedValue` cannot be used for binary data.
 Per [[!RFC8259]] [Section 8.2](https://www.rfc-editor.org/rfc/rfc8259.html#section-8.2), using escape sequences that cannot encode Unicode characters to represent binary data is not portable and may cause runtime errors.
 Therefore, data formats that are not always representable as Unicode code points SHOULD use `externalSerializedValue`.