Help: Document VS_DEBUGGER_WORKING_DIRECTORY supported VS versions

Help/prop_tgt/VS_DEBUGGER_WORKING_DIRECTORY.rst

@@ -4,3 +4,6 @@ VS_DEBUGGER_WORKING_DIRECTORY
 Sets the local debugger working directory for Visual Studio C++ targets.
 This is defined in ``<LocalDebuggerWorkingDirectory>`` in the Visual Studio
 project file.
+
+This property only works for Visual Studio 2010 and above;
+it is ignored on other generators.