Add build-only diagnostic note to CS0169 documentation

Co-authored-by: BillWagner <493969+BillWagner@users.noreply.github.com>

Author: Copilot

docs/csharp/misc/cs0169.md

@@ -13,6 +13,9 @@ ms.assetid: 04b0015f-658d-440a-b9ba-831178f1a180
 The private field 'class member' is never used  
 
  A private variable was declared but never referenced. A common way to generate this warning is when you declare a private member of a class and do not use it.  
+
+> [!NOTE]
+> This warning is only reported during explicit **Build** or **Rebuild** operations. It does not appear during typing in the IDE as part of IntelliSense diagnostics. This means that if you fix the warning by using the field or removing it, the warning might persist in the error list until you build or rebuild the project again.
   
  The following sample generates CS0169: