Add note about XSLT 3.0 aberration

specifications/xpath-functions-40/src/function-catalog.xml

@@ -23092,6 +23092,11 @@ map:of-pairs($maps =!> map:pairs(),
             types. The type of a map (for example <code>map(xs:integer, xs:string)</code>) is
             descriptive of the entries it currently contains, but is not a constraint on how the map
             may be combined with other maps.</p>
+         <p>The XSLT 3.0 recommendation included a specification of this function that incorrectly used
+            the option value <code>{'duplicates':'unspecified'}</code> in place of
+            <code>{'duplicates':'use-any'}</code>. XSLT implementations wishing to
+            preserve backwards compatibility <rfc2119>may</rfc2119> choose to retain support
+            for this setting.</p>
       </fos:notes>
       <fos:examples>
          <fos:variable name="week" id="v-map-merge-week"
@@ -24650,7 +24655,20 @@ else map:put($map, $key, $action(()))
   return map:pair($key, $val)
 ) => map:of-pairs($options)      
       </fos:equivalent>
-
+      <fos:errors>
+         <p>An error is raised <errorref spec="FO" class="RG" code="0013"
+               /> if both the <code>combine</code> and <code>duplicates</code>
+            options are present.</p>
+
+         <p>An error is raised <errorref spec="FO" class="JS" code="0003"
+               /> if the value of 
+          <code>$options</code> indicates that duplicates are to be rejected, and a duplicate key is encountered.</p>
+         <p>An error is raised <errorref spec="FO" class="JS" code="0005"
+               /> if the value of 
+          <code>$options</code> includes an entry whose key is defined 
+          in this specification, and whose value is not a permitted value for that key.</p>
+
+      </fos:errors>
 
       <fos:notes>