Clarify that outcome takes Promises* and Channels*

As in zero or more

Author: lizmat

S17-concurrency.pod

@@ -424,8 +424,9 @@ is assumed between the two.)
 =head1 Outcome
 
 There is an C<outcome> statement [keywords still negotiable].  In its
-simplest form it just takes a list of C<Promise>s and C<Channel>s and returns
-a lazy list of all the values returned by all Promises and Channels specified:
+simplest form it just takes a list of C<Promise>s and / or C<Channel>s and
+returns a lazy list of all the values returned by all Promises and Channels
+specified:
 
     my @values = outcome @p, @c;