Add details for DOTJ068E circular keyref error

Per dita-ot/dita-ot#2227

user-guide/DITA-messages-details.xml

@@ -327,6 +327,17 @@
           the extensions "dita" and "xml" for DITA topics, as mandated by the DITA Specification. Please verify that
           your topics use one of these extensions, or configure the toolkit to allow additional extensions.</stentry>
       </strow>
+      <strow>
+        <stentry conref="DITA-messages.xml#msgs/DOTJ068E-extra" conaction="pushreplace">
+          <p>A circular reference was found in key definitions: a series of key references where the last key references
+            the first.</p>
+          <p>This may occur if a <xmlelement>topicref</xmlelement> element contains both a key name in the
+              <xmlatt>keys</xmlatt> attribute and a reference to the same key in the <xmlatt>keyref</xmlatt> attribute,
+            or if a <xmlatt>keyref</xmlatt> attribute points to a key that refers back to the referencing element.</p>
+          <p>To resolve this issue, change the target of the <xmlatt>keyref</xmlatt> so the key is defined by pointing
+            to a resource other than itself.</p>
+        </stentry>
+      </strow>
       <strow>
         <stentry conref="DITA-messages.xml#msgs/DOTX001W-extra" conaction="pushreplace">This build uses generated text,
           such as the phrase "Related information" (which is generated above many link groups). The toolkit was unable