Help: Clarify the default value for test property COST

Co-Authored-By: Steve Mokris <[email protected]>

Help/prop_test/COST.rst

@@ -1,8 +1,14 @@
 COST
 ----
 
-Set this to a floating point value. Tests in a test set will be run
-in descending order of cost.
+This property describes the cost of a test.  When parallel testing is
+enabled, tests in the test set will be run in descending order of cost.
+Projects can explicitly define the cost of a test by setting this property
+to a floating point value.
 
-This property describes the cost of a test.  You can explicitly set
-this value; tests with higher ``COST`` values will run first.
+When the cost of a test is not defined by the project,
+:manual:`ctest <ctest(1)>` will initially use a default cost of ``0``.
+It computes a weighted average of the cost each time a test is run and
+uses that as an improved estimate of the cost for the next run.  The more
+a test is re-run in the same build directory, the more representative the
+cost should become.