Clarify that whenever uses .act, not .tap

...and so its block is executed just one at a time.

smls++ for pointing that out.

doc/Language/concurrency.pod6

@@ -312,8 +312,9 @@ This could also be written using the C<react> keyword:
         }
     }
 
-Here the X<C<whenever>> keyword creates a tap on the L<Supply> from the provided
-block. The C<react> block is exited when C<done()> is called in one of the taps.
+Here the X<C<whenever>> keyword uses L«C<.act>|/type/Supply#method_act»
+to create a tap on the L<Supply> from the provided block. The C<react>
+block is exited when C<done()> is called in one of the taps.
 
 A second argument can be supplied to C<interval> which specifies a delay
 in seconds before the first event is fired. Each tap of a supply created