add guidance for using aria-description

src/accessibility.html

@@ -424,6 +424,37 @@ <h5 id="acc_natural_lang">Natural-language Mathematics</h5>
 
 </section>
 
+<section>
+  <h5  id="acc_descriptions">Accessible Descriptions</h5>
+
+<p>It is sometimes beneficial to a reader to have additional, auxiliary information for a given
+   mathematical construct. This is particularly the case in educational materials where
+   newly introduced syntax benefits from repeated reinforcement. Such information is often
+   too verbose for more familiar readers, indeed even to the same person after their first reading.
+   Hence, common AT behavior has been to only vocalize an
+   <a href="https://www.w3.org/TR/accname-1.2/#dfn-accessible-description">accessible description</a>,
+   on user request, omitting it by default.</p>
+<p>It is appropriate to provide such descriptions using the ARIA 1.3 attribute 
+  <a href="https://www.w3.org/TR/wai-aria-1.3/#aria-description"><code class="attribute">aria-description</code></a>,
+  which allows for a literal string value to annotate its host element.
+  As an example, consider an equation which describes the types of all variables.
+</p>
+
+<div class="example mathml mmlcore">
+<pre>
+&lt;mrow aria-description="circumference of a circle"&gt;
+  &lt;mn&gt;2&lt;/mn&gt;
+  &lt;mi aria-description="mathematical constant"&gt;π&lt;/mi&gt;
+  &lt;mi aria-description="radius variable"&gt;r&lt;/mi&gt;
+&lt;/mrow&gt;
+</pre>
+</div>
+
+<p>While <code class="attribute">aria-description</code> has been used on its own for brevity,
+  it is recommended to treat it as a secondary add-on which follows
+  a primary <code class="attribute">intent</code> annotation, as appropriate.
+</p>
+</section>
    </section>
   </section>
  </section>