Document that responsive tables use overflow-y: hidden

Closes twbs#15288

[skip sauce]

docs/_includes/css/tables.html

@@ -351,6 +351,10 @@ <h4>Conveying meaning to assistive technologies</h4>
   <h2 id="tables-responsive">Responsive tables</h2>
   <p>Create responsive tables by wrapping any <code>.table</code> in <code>.table-responsive</code> to make them scroll horizontally on small devices (under 768px). When viewing on anything larger than 768px wide, you will not see any difference in these tables.</p>
 
+  <div class="bs-callout bs-callout-warning" id="callout-tables-responsive-overflow">
+    <h4>Vertical clipping/truncation</h4>
+    <p>Responsive tables make use of <code>overflow-y: hidden</code>, which clips off any content that goes beyond the bottom or top edges of the table. In particular, this can clip off dropdown menus and other third-party widgets.</p>
+  </div>
   <div class="bs-callout bs-callout-warning" id="callout-tables-responsive-ff-fieldset">
     <h4>Firefox and fieldsets</h4>
     <p>Firefox has some awkward fieldset styling involving <code>width</code> that interferes with the responsive table. This cannot be overriden without a Firefox-specific hack that we <strong>don't</strong> provide in Bootstrap:</p>