Help: Update documentation for PROCESSORS test property

This updates the documentation for the PROCESSORS test property to show
how it will affect the summary output for labels and sub projects.

Help/prop_test/PROCESSORS.rst

@@ -1,8 +1,13 @@
 PROCESSORS
 ----------
 
-How many process slots this test requires
+Set to specify how many process slots this test requires.
 
 Denotes the number of processors that this test will require.  This is
 typically used for MPI tests, and should be used in conjunction with
-the ctest_test PARALLEL_LEVEL option.
+the :command:`ctest_test` ``PARALLEL_LEVEL`` option.
+
+This will also be used to display a weighted test timing result in label and
+subproject summaries in the command line output of :manual:`ctest(1)`. The wall
+clock time for the test run will be multiplied by this property to give a
+better idea of how much cpu resource CTest allocated for the test.