Allow img element to be a child of picture element

Previously picture element specified that it contains source elements
and one img element, but only source element specified that it can
appear as a child of picture element. Now img element also explicitly
allows this.

This is especially useful for automated tooling which might scrape
through the document and expect a parent-child relationship to be
mutual.

source

@@ -28546,6 +28546,7 @@ interface <dfn interface>HTMLSourceElement</dfn> : <span>HTMLElement</span> {
    <dd><span>Palpable content</span>.</dd>
    <dt><span data-x="concept-element-contexts">Contexts in which this element can be used</span>:</dt>
    <dd>Where <span>embedded content</span> is expected.</dd>
+   <dd>As a child of a <code>picture</code> element, after all <code>source</code> elements.</dd>
    <dt><span data-x="concept-element-content-model">Content model</span>:</dt>
    <dd><span data-x="concept-content-nothing">Nothing</span>.</dd>
    <dt><span data-x="concept-element-attributes">Content attributes</span>:</dt>
@@ -139347,6 +139348,7 @@ INSERT INTERFACES HERE
   何志翔 (HE Zhixiang),
   Henri Sivonen,
   Henrik Lied,
+  Henrik Lievonen,
   Henry Lewis,
   Henry Mason,
   Henry Story,