Help: Add notes for topic 'timeout_after_match'

Help/release/dev/timeout_after_match.rst

@@ -0,0 +1,6 @@
+timeout_after_match
+-------------------
+
+* CTest learned to optionally enforce a secondary timeout after matching
+  certain output from a test.  See the :prop_test:`TIMEOUT_AFTER_MATCH` test
+  property.