Explain the 4-space HTML line-wrapping guidance

This clarification was added to the internal style guide a few months ago.

htmlcssguide.html

@@ -420,7 +420,8 @@ instead of an error.)</p>
 wrapping long lines if it significantly improves readability.</p>
 
 <p>When line-wrapping, each continuation line should be indented at least 4
-additional spaces from the original line.</p>
+additional spaces from the original line to distinguish wrapped attributes from
+child elements.</p>
 
 <pre><code class="language-html good">&lt;md-progress-circular md-mode="indeterminate" class="md-accent"
     ng-show="ctrl.loading" md-diameter="35"&gt;