Fix ProjectSettings.save_custom documentation

(cherry picked from commit 728c0c1e14d13f7aa1f48e3b110bee9b6fcc6d77)

doc/classes/ProjectSettings.xml

@@ -137,7 +137,7 @@
 			<argument index="0" name="file" type="String">
 			</argument>
 			<description>
-				Saves the configuration to a custom file.
+				Saves the configuration to a custom file. The file extension must be [code].godot[/code] (to save in text-based [ConfigFile] format) or [code].binary[/code] (to save in binary format).
 			</description>
 		</method>
 		<method name="set_initial_value">