Define "overridden constructor steps"

This is for use in [HTMLConstructor] in whatwg/html#4915.

Author: domenic

index.bs

@@ -11131,13 +11131,25 @@ and is described in more detail in [[#interface-prototype-object]].
 Note: Since an [=interface object=] is a [=function object=]
 the <code>typeof</code> operator will return "function" when applied to an interface object.
 
+An interface may have <dfn export>overridden constructor steps</dfn>, which can
+change the behavior of the [=interface object=] when called or constructed. By
+default interfaces do not have such steps.
+
+<p class="advisement">
+    In general, constructors are described by defining a [=constructor operation=] and its behavior.
+    The [=overridden constructor steps=] are used only for more complicated situations.
+    Editors who wish to use this feature are strongly advised to discuss this by
+    <a href="https://github.com/heycam/webidl/issues/new?title=Intent%20to%20use%20constructor%20steps">filing an issue</a> before proceeding.
+</p>
+
 <div algorithm>
 
     The [=interface object=] for a given [=interface=] |I|
     with [=identifier=] |id| and in [=Realm=] |realm|
     is <dfn lt="create an interface object">created</dfn> as follows:
 
-    1.  Let |steps| be the following steps:
+    1.  Let |steps| be |I|'s [=overriden constructor steps=] if they exist, or
+        the following steps otherwise:
         1.  If |I| was not declared with a [=constructor operation=],
             then [=ECMAScript/throw=] a {{ECMAScript/TypeError}}.
         1.  If {{NewTarget}} is <emu-val>undefined</emu-val>, then