Clarify that subtest() counts as one test

doc/Language/testing.pod6

@@ -437,8 +437,9 @@ The result of a group of subtests is only C<ok> if all subtests are C<ok>.
 =item X<subtest(&subtests, $description?)|subtest>
 
 The C<subtest> function executes the given block, consisting of usually more
-than one test, possibly including a C<plan> or C<done>.  It will pass the
-test only if B<all> tests in the block, pass.  The function accepts an
+than one test, possibly including a C<plan> or C<done>, and counts as I<one>
+test in C<plan>, C<todo>, or C<skip> counts. It will pass the
+test only if B<all> tests in the block, pass. The function accepts an
 optional C<$description> of the test.
 
 =begin code