add preference help for custom folding regions

See eclipse-jdt/eclipse.jdt.ui#1825

eclipse.platform.common/bundles/org.eclipse.jdt.doc.user/reference/preferences/java/editor/ref-preferences-folding.htm

@@ -91,8 +91,45 @@ <h1>Java Editor Folding Preferences</h1>
 			<p>On</p>
 			</td>
 		</tr>
+		<tr>
+			<td valign="top">
+			<p>Custom Folding regions</p>
+			</td>
+			<td valign="top">
+			<p>If enabled, custom folding regions are folded when opening a new editor.</p>
+			</td>
+			<td valign="top">
+			<p>Off</p>
+			</td>
+		</tr>
+		<tr>
+			<td valign="top">
+			<p>Text marking start of region</p>
+			</td>
+			<td valign="top">
+			<p>Comments starting with the content of this option indicate the start of custom folding regions.</p>
+			</td>
+			<td valign="top">
+			<p>#region</p>
+			</td>
+		</tr>
+		<tr>
+			<td valign="top">
+			<p>Text marking end of region</p>
+			</td>
+			<td valign="top">
+			<p>Comments starting with the content of this option indicate the end of custom folding regions.</p>
+			</td>
+			<td valign="top">
+			<p>#endregion</p>
+			</td>
+		</tr>
 	</tbody>
 </table>
+<p>
+Custom folding regions allow creating regions to fold using a marker text for the start and end of the region in a comment.
+Leaving the fields for the start or end markers empty disables this feature.
+</p>
 <h3 class="related">Related References</h3>
 <a href="../ref-preferences-editor.htm">Java Editor preference page</a>
 </body>