document it

Author: mdo

docs/4.0/utilities/position.md

@@ -26,6 +26,8 @@ Position an element at the bottom of the viewport, from edge to edge. Be sure yo
 
 Position an element at the top of the viewport, from edge to edge, but only after you scroll past it. The `.sticky-top` utility uses CSS's `position: sticky`, which isn't fully supported in all browsers.
 
+**Microsoft Edge and IE11 will render `position: sticky` as `position: relative`.** As such, we wrap the styles in a `@supports` query, limiting the stickiness to only browsers that properly can render it.
+
 {% highlight html %}
 <div class="sticky-top">...</div>
 {% endhighlight %}