Clarify fragment-only vs all URIs

Fix this wording to make it more clear that this requirement
is about fragment-only URI references, and that other sorts
of URI references are not constrained by this paragraph.

Author: handrews

jsonschema-core.xml

@@ -470,7 +470,7 @@
                     Pointer references are updated.
                 </t>
                 <t>
-                    The effect of defining a URI fragment "$id" that neither
+                    The effect of defining a fragment-only "$id" URI reference that neither
                     matches the above requirements nor is a valid JSON pointer
                     is not defined.
                 </t>