Help: Document meaning of TIMEOUT test property with value 0

Document the behavior added by commit 51bb493574 (Test TIMEOUT property
explicitly set to zero should be honored, 2011-01-03, v2.8.4~118^2).

Help/prop_test/TIMEOUT.rst

@@ -7,3 +7,6 @@ This property if set will limit a test to not take more than the
 specified number of seconds to run.  If it exceeds that the test
 process will be killed and ctest will move to the next test.  This
 setting takes precedence over :variable:`CTEST_TEST_TIMEOUT`.
+
+An explicit ``0`` value means the test has no timeout, except as
+necessary to honor :option:`ctest --stop-time`.