Mention (and discourage) plan *

Closes #4640

Author: coke

doc/Type/Test.rakudoc

@@ -29,6 +29,9 @@ the subtest.
 If a C<plan> is used, it's not necessary to specify the end of testing with
 C<done-testing>.
 
+I<Note that C<plan *> is allowed, is the same as not declaring a plan, and is
+not recommended.>
+
 You can also provide a C<:skip-all> named argument instead of a test count,
 to indicate that you want to skip all of the tests. Such a plan will
 call L«C<exit>|/routine/exit», unless used inside of a C<subtest>.