Docs: Add popups dismissable attribute to the reference

Author: MauriceG

docs/api/data-attributes.html

@@ -491,6 +491,11 @@ <h2><a href="../pages/popup/">Popup</a></h2>
 					<th>data-corners</th>
 					<td><strong>true</strong> | false</td>
 				</tr>
+				<tr>
+					<th>data-dismissable</th>
+					<td><strong>true</strong> | false - If set to false prevents popup closing by clicking outside of the popup or pressing the Escape key</td>
+				</tr>
+
 				<tr>
 					<th>data-overlay-theme</th>
 					<td>swatch letter (a-z) - Default "null" (transparent background)</td>