Add advisement for LegacyUnenumerableNamedProperties

index.bs

@@ -8527,6 +8527,31 @@ entails.
 
 <h4 id="LegacyUnenumerableNamedProperties" extended-attribute lt="LegacyUnenumerableNamedProperties">[LegacyUnenumerableNamedProperties]</h4>
 
+<div class="advisement">
+
+    The [{{LegacyUnenumerableNamedProperties}}] [=extended attribute=] is an undesirable feature.
+    It exists only so that legacy Web platform features can be specified.
+    It should not be used in specifications
+    unless required to specify the behavior of legacy APIs.
+    Editors who wish to use this feature are strongly advised to discuss this
+    by <a href="https://github.com/w3c/respec/issues/new?title=[legacy]">filing an issue</a>
+    before proceeding.
+
+    <small class="non-normative">
+        The [{{LegacyUnenumerableNamedProperties}}] [=extended attribute=] appears on
+        the following [=interfaces=]:
+        {{HTMLCollection}},
+        {{NamedNodeMap}},
+        {{HTMLAllCollection}},
+        {{HTMLFormElement}},
+        {{PluginArray}},
+        {{MimeTypeArray}},
+        {{Plugin}}, and
+        {{Window}}. [[DOM]] [HTML]
+    </small>
+
+</div>
+
 If the [{{LegacyUnenumerableNamedProperties}}]
 [=extended attribute=]
 appears on a [=interface=]