Add example for unsupported_extensions value.

draft-ietf-ppm-dap.md

@@ -1295,7 +1295,9 @@ Leader does abort for this reason, it MAY indicate the unsupported extensions in
 the resulting problem document via an extension member ({{Section 3.2 of
 !RFC9457}}) "unsupported_extensions" on the problem document; this member MUST
 contain an array of numeric values indicating the extension code points which
-were not recognized.
+were not recognized. For example, if the report upload contained two unsupported
+extensions with code points `23` and `42`, the "unsupported_extensions" member
+would contain the value `[23, 42]`.
 
 If the Leader's input share contains two extensions with the same type, then the
 Leader MAY abort the upload request with error "invalidMessage".