Document use of spaces in ConfigFile

doc/classes/ConfigFile.xml

@@ -25,6 +25,7 @@
 		    # Save the changes by overwriting the previous file
 		    config.save("user://settings.cfg")
 		[/codeblock]
+		Keep in mind that section and property names can't contain spaces. Anything after a space will be ignored on save and on load.
 	</description>
 	<tutorials>
 	</tutorials>