Add note about automatic window title to FileDialog documentation.

doc/classes/FileDialog.xml

@@ -4,7 +4,7 @@
 		Dialog for selecting files or directories in the filesystem.
 	</brief_description>
 	<description>
-		FileDialog is a preset dialog used to choose files and directories in the filesystem. It supports filter masks.
+		FileDialog is a preset dialog used to choose files and directories in the filesystem. It supports filter masks. The FileDialog automatically sets its window title according to the [member file_mode]. If you want to use a custom title, disable this by setting [member mode_overrides_title] to [code]false[/code].
 	</description>
 	<tutorials>
 	</tutorials>