Help: Fix wrong default for WORKING_DIRECTORY test property

Help/prop_test/WORKING_DIRECTORY.rst

@@ -3,5 +3,7 @@ WORKING_DIRECTORY
 
 The directory from which the test executable will be called.
 
-If this is not set it is called from the directory the test executable
-is located in.
+If this is not set, the test will be run with the working directory set to the
+binary directory associated with where the test was created (i.e. the
+:variable:`CMAKE_CURRENT_BINARY_DIR` for where :command:`add_test` was
+called).