GDScript: Fix @icon annotation

modules/gdscript/doc_classes/@GDScript.xml

@@ -563,6 +563,7 @@
 				[/codeblock]
 				[b]Note:[/b] Only the script can have a custom icon. Inner classes are not supported.
 				[b]Note:[/b] As annotations describe their subject, the [code]@icon[/code] annotation must be placed before the class definition and inheritance.
+				[b]Note:[/b] Unlike other annotations, the argument of the [code]@icon[/code] annotation must be a string literal (constant expressions are not supported).
 			</description>
 		</annotation>
 		<annotation name="@onready">