clarify and danger-ify affix docs usage requirements, per #10235

docs/_includes/js/affix.html

@@ -7,7 +7,7 @@
   <hr class="bs-docs-separator">
 
   <h2 id="affix-usage">Usage</h2>
-  <p>Use the affix plugin via data attributes or manually with your own JavaScript. <strong>In both situations, you must provide CSS for the positioning of your content.</strong></p>
+  <p>Use the affix plugin via data attributes or manually with your own JavaScript. <strong class="text-danger">In both situations, you must provide CSS for the positioning and width of your affixed content.</strong></p>
 
   <h3>Positioning via CSS</h3>
   <p>The affix plugin toggles between three classes, each representing a particular state: <code>.affix</code>, <code>.affix-top</code>, and <code>.affix-bottom</code>. You must provide the styles for these classes yourself (independent of this plugin) to handle the actual positions.</p>