Document behaviour of TODOed subtest

Author: zoffixznet

doc/Language/testing.pod6

@@ -699,6 +699,9 @@ The result from the test code above will be something like:
     #      got: '3'
 =end code
 
+Note that if you C<todo> a C<subtest>, all of the failing tests inside of it will be
+automatically marked TODO as well and will I<not> count towards your original TODO count.
+
 =item X<skip($reason, $count = 1)|skip>
 
 Skip C<$count> tests, giving a C<$reason> as to why.  By default only one