Help developers better understand self-closing-tag syntax

* Clarify that the solidus character in an HTML-element start tag doesn’t mark the HTML element as self-closing.

* Explicitly caution developers that the solidus character, if directly preceded by an unquoted attribute value, becomes part of the attribute value rather than being discarded by the parser.

source

@@ -106626,8 +106626,12 @@ dictionary <dfn dictionary>StorageEventInit</dfn> : <span>EventInit</span> {
 
    <li>Then, if the element is one of the <span>void elements</span>, or if the element is a <span
    data-x="foreign elements">foreign element</span>, then there may be a single U+002F SOLIDUS
-   character (/). This character has no effect on <span>void elements</span>, but on <span>foreign
-   elements</span> it marks the start tag as self-closing.</li>
+   character (/), which on <span>foreign elements</span> marks the start tag as self-closing. On
+   <span>void elements</span>, it does not mark the start tag as self-closing but instead is
+   unnecessary and has no effect of any kind. For such void elements, it should be used only with
+   caution — especially since, if directly preceded by an <a href="#unquoted">unquoted attribute
+   value</a>, it becomes part of the attribute value rather than being discarded by the
+   parser.</li>.
 
    <li>Finally, start tags must be closed by a U+003E GREATER-THAN SIGN character (&gt;).</li>
   </ol>