Add obj/bin deletion solution to CS0518 documentation

Co-authored-by: BillWagner <[email protected]>

Author: Copilot

docs/csharp/language-reference/compiler-messages/cs0518.md

@@ -30,6 +30,8 @@ The main cause for this problem is that the project is not importing mscorlib.dl
 
 - Make sure that the project refers to the correct mscorlib.dll.  
 
+- Close Visual Studio, delete the `obj` and `bin` folders from your project directory, then reopen Visual Studio and rebuild the project.
+  
 - Reinstall the .NET Framework common language runtime (if the previous solutions do not solve the problem).
 
 Optionally