Core: Add note on multiple keys in an object

Author: awwright

jsonschema-core.xml

@@ -193,7 +193,7 @@
                         <t>both are strings, and are the same codepoint-for-codepoint; or</t>
                         <t>both are numbers, and have the same mathematical value; or</t>
                         <t>both are arrays, and have an equal value item-for-item; or</t>
-                        <t>both are objects, and each property in one has exactly one property with an equal key the other, and that other property has an equal value.</t>
+                        <t>both are objects, and each property in one has exactly one property with an equal key the other, and that other property has an equal value. No way is provided for a key to be used multiple times.</t>
                     </list>
                 </t>
             </section>