Update mstest0011.md (dotnet#41914)

Author: engyebrahim

docs/core/testing/mstest-analyzers/mstest0011.md

@@ -39,6 +39,7 @@ Methods marked with `[ClassCleanup]` should follow the following layout to be va
 - it should not be generic
 - it should not take any parameter
 - return type should be `void`, `Task` or `ValueTask`
+- `InheritanceBehavior.BeforeEachDerivedClass` attribute parameter should be specified if the class is `abstract`.
 
 [!INCLUDE [test-class-rules](includes/test-class-rules.md)]