Affix docs: add MDN link for `position: sticky`

docs/_includes/js/affix.html

@@ -2,7 +2,7 @@
   <h1 id="affix" class="page-header">Affix <small>affix.js</small></h1>
 
   <h2 id="affix-examples">Example</h2>
-  <p>The affix plugin toggles <code>position: fixed;</code> on and off emulating the effect found with <code>position: sticky;</code>. The subnavigation on the right is a live demo of the affix plugin.</p>
+  <p>The affix plugin toggles <code>position: fixed;</code> on and off, emulating the effect found with <a href="https://developer.mozilla.org/en-US/docs/Web/CSS/position#Sticky_positioning"><code>position: sticky;</code></a>. The subnavigation on the right is a live demo of the affix plugin.</p>
 
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