Explains you shouldn't use last in whenever

That would close #1551. Please reopen if you don't feel happy about it.

Author: JJ

doc/Language/concurrency.pod6

@@ -319,7 +319,7 @@ This could also be written using the C<react> keyword:
 
 Here the 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.
+block is exited when C<done()> is called in one of the taps. Using C<last> to exit the block would produce an error indicating that it's not really a loop construct.
 
 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