Editorial: say which <source> attributes are allowed where

Fixes #6415.

source

@@ -26028,13 +26028,13 @@ interface <dfn>HTMLPictureElement</dfn> : <span>HTMLElement</span> {
    <dd><span data-x="concept-content-nothing">Nothing</span>.</dd>
    <dt><span data-x="concept-element-attributes">Content attributes</span>:</dt>
    <dd><span>Global attributes</span></dd>
-   <dd><code data-x="attr-source-src">src</code></dd>
    <dd><code data-x="attr-source-type">type</code></dd>
-   <dd><code data-x="attr-source-srcset">srcset</code></dd>
-   <dd><code data-x="attr-source-sizes">sizes</code></dd>
-   <dd><code data-x="attr-source-media">media</code></dd>
-   <dd><code data-x="attr-dim-width">width</code></dd>
-   <dd><code data-x="attr-dim-height">height</code></dd>
+   <dd><code data-x="attr-source-src">src</code> (in <code>video</code> or <code>audio</code>)</dd>
+   <dd><code data-x="attr-source-srcset">srcset</code> (in <code>picture</code>)</dd>
+   <dd><code data-x="attr-source-sizes">sizes</code> (in <code>picture</code>)</dd>
+   <dd><code data-x="attr-source-media">media</code> (in <code>picture</code>)</dd>
+   <dd><code data-x="attr-dim-width">width</code> (in <code>picture</code>)</dd>
+   <dd><code data-x="attr-dim-height">height</code> (in <code>picture</code>)</dd>
    <dt><span
    data-x="concept-element-accessibility-considerations">Accessibility considerations</span>:</dt>
    <dd><a href="https://w3c.github.io/html-aria/#el-source">For authors</a>.</dd>
@@ -120440,7 +120440,7 @@ interface <dfn>MimeType</dfn> {
           <code data-x="attr-dim-height">img</code>;
           <code data-x="attr-dim-height">input</code>;
           <code data-x="attr-dim-height">object</code>;
-          <code data-x="attr-dim-width">source</code>;
+          <code data-x="attr-dim-height">source</code> (in <code>picture</code>);
           <code data-x="attr-dim-height">video</code>
      <td> Vertical dimension
      <td> <span>Valid non-negative integer</span>
@@ -120617,7 +120617,7 @@ interface <dfn>MimeType</dfn> {
     <tr>
      <th> <code data-x="">media</code>
      <td> <code data-x="attr-link-media">link</code>;
-          <code data-x="attr-source-media">source</code>;
+          <code data-x="attr-source-media">source</code> (in <code>picture</code>);
           <code data-x="attr-style-media">style</code>
      <td> Applicable media
      <td> <span>Valid media query list</span>
@@ -120895,7 +120895,7 @@ interface <dfn>MimeType</dfn> {
           <code data-x="attr-img-src">img</code>;
           <code data-x="attr-input-src">input</code>;
           <code data-x="attr-script-src">script</code>;
-          <code data-x="attr-source-src">source</code>;
+          <code data-x="attr-source-src">source</code> (in <code>video</code> or <code>audio</code>);
           <code data-x="attr-track-src">track</code>;
           <code data-x="attr-media-src">video</code>
      <td> Address of the resource
@@ -121070,7 +121070,7 @@ interface <dfn>MimeType</dfn> {
           <code data-x="attr-dim-width">img</code>;
           <code data-x="attr-dim-width">input</code>;
           <code data-x="attr-dim-width">object</code>;
-          <code data-x="attr-dim-width">source</code>;
+          <code data-x="attr-dim-width">source</code> (in <code>picture</code>);
           <code data-x="attr-dim-width">video</code>
      <td> Horizontal dimension
      <td> <span>Valid non-negative integer</span>