Clarify what happens when th scope attribute is not specified (mdn#368)

This change refines the description of the "scope" attribute of the "th"
element to explain what happens when no "scope" attribute is specified
at all (what the HTML spec calls the "auto" state).

The change also corrects an existing error: "auto" isn’t actually an
allowed value for the "scope" attribute; instead "auto" is just an
abstract/internal value which the spec requires tools to use in the case
where there’s no explicit value, or the value is invalid.

Fixes mdn#362

files/en-us/web/html/element/th/index.html

@@ -78,10 +78,8 @@ <h2 id="Attributes">Attributes</h2>
   <li><code>col</code>: The header relates to all cells of the column it belongs to.</li>
   <li><code>rowgroup</code>: The header belongs to a rowgroup and relates to all of its cells. These cells can be placed to the right or the left of the header, depending on the value of the <code><a href="/en-US/docs/Web/HTML/Global_attributes/dir">dir</a></code> attribute in the {{HTMLElement("table")}} element.</li>
   <li><code>colgroup</code>: The header belongs to a colgroup and relates to all of its cells.</li>
-  <li><code>auto</code></li>
  </ul>
-
- <p>The default value when this attribute is not specified is <code>auto</code>.</p>
+ If the <code>scope</code> attribute is not specified, or its value is not <code>row</code>, <code>col</code>, or <code>rowgroup</code>, or <code>colgroup</code>, then browsers automatically select the set of cells to which the header cell applies.
  </dd>
 </dl>