Clarify that the namespace check in "create an element" is redundant

Fixes whatwg/html#1402.

Author: domenic

dom.bs

@@ -5732,8 +5732,16 @@ given a <var>document</var>, <var>localName</var>, <var>namespace</var>, and opt
      <li><p>If <var>result</var>'s <a>node document</a> is not <var>document</var>, then
      <a>throw</a> a {{NotSupportedError}}.
 
-     <li><p>If <var>result</var>'s <a for=Element>namespace</a> is not the <a>HTML namespace</a>,
-     then <a>throw</a> a {{NotSupportedError}}.
+     <li>
+      <p>If <var>result</var>'s <a for=Element>namespace</a> is not the <a>HTML namespace</a>,
+      then <a>throw</a> a {{NotSupportedError}}.
+
+      <p class="note">As of the time of this writing, every element that implements the
+      {{HTMLElement}} interface is also in the <a>HTML namespace</a>, so this check is currently
+      redundant with the above brand check. However, this is not guaranteed to be true forever in
+      the face of potential specification changes, such as converging certain SVG and HTML
+      interfaces.
+     </li>
 
      <li><p>If <var>result</var>'s <a for=Element>local name</a> is not equal to
      <var>localName</var>, then <a>throw</a> a {{NotSupportedError}}.