add caution about done-testing

Author: tbrowder

doc/Language/testing.pod6

@@ -178,7 +178,8 @@ C<done-testing>.
 
 Specify that testing has finished.  Use this function when you do not as yet
 have a C<plan> for the number of tests to run.  A C<plan> is thus not
-required when using C<done-testing>.
+required when using C<done-testing>. (Note:  It is recommended that the C<done-testing>
+be removed when all tests are finalized to help catch future test errors.)
 
 =head1 Testing return values