Add mention that only one whenever block runs at a time

Author: lizmat

doc/Language/concurrency.pod6

@@ -343,6 +343,7 @@ In this example we are watching two supplies.
      $bread-supplier.emit("Thick sliced"); # OUTPUT: «We've got bread: Thick sliced␤»
      $vegetable-supplier.emit("Lettuce");  # OUTPUT: «We've got a vegetable: Lettuce␤»
 
+Please note that one should keep the code inside the C<whenever> as small as possible, as only one C<whenever> block will be executed at any time.  One can use a C<start> block inside the C<whenever> block to run longer running code.
 
 =head3 X<C<react>|react>