Update gap.html (#1110)

Additionally, <code>.is-variable</code> should be added on the <code>.columns</code> container.

docs/documentation/columns/gap.html

@@ -194,6 +194,9 @@ doc-subtab: gap
           <code>is-8</code> is the maximum gap of <code>2rem</code>
         </li>
       </ul>
+      <p>
+        Additionally, <code>.is-variable</code> should be added on the <code>.columns</code> container.
+      </p>
     </div>
 
     {% include klmn.html %}