Make callbacks and dictionary-like distinguishable

Fixes #1191.

Author: vmpstr

index.bs

@@ -3467,7 +3467,7 @@ the following algorithm returns <i>true</i>.
             <td class="belowdiagonal">
             <td class="belowdiagonal">
             <td>
-            <td>
+            <td>(c)
             <td>●
         <tr>
             <th>dictionary-like
@@ -3505,6 +3505,16 @@ the following algorithm returns <i>true</i>.
             <a href="#limit-bigint-numeric-overloading">a separate restriction on their use in
             overloading</a> below. Please also note <a href="#limit-bigint-numeric-unions">the
             advice about using unions of these types</a>.
+        1.  A [=callback function=] that does not have
+            [{{LegacyTreatNonObjectAsNull}}] extended attribute is
+            distinguishable from a type in the dictionary-like category.
+            <div class="note">
+              For example, when converting an ECMAScript value to [=union
+              type=] which includes a [=callback function=] and a
+              dictionary-like type, if the value is [=ECMAScript/callable=],
+              then it is converted to a [=callback function=]. Otherwise, it is
+              converted to a dictionary-like type.
+            </div>
     </ol>
 
     <div class="example" id="example-distinguishability-diff-types">