Fix C# diagnostic messages to match 4.2

tutorials/scripting/c_sharp/diagnostics/GD0001.rst

@@ -1,5 +1,5 @@
-GD0001: Missing partial modifier on declaration of type which is a subclass of GodotObject
-==========================================================================================
+GD0001: Missing partial modifier on declaration of type that derives from GodotObject
+=====================================================================================
 
 ====================================  ======================================
                                       Value

tutorials/scripting/c_sharp/diagnostics/GD0002.rst

@@ -1,5 +1,5 @@
-GD0002: Missing partial modifier on declaration of type which contains one or more subclasses of GodotObject
-============================================================================================================
+GD0002: Missing partial modifier on declaration of type which contains nested classes that derive from GodotObject
+==================================================================================================================
 
 ====================================  ======================================
                                       Value