Add warning about gather/take react/whenever

nine++

Closes #4041

Author: coke

doc/Language/control.pod6

@@ -759,6 +759,11 @@ makes it a hash.
     my %h = gather { take "foo" => 1; take "bar" => 2};
     say %h;                                   # OUTPUT: «{bar => 2, foo => 1}␤»
 
+B<Note>: C<gather/take> must not be used to collect results from C<react/whenever>.
+The C<whenever> block is not run from the thread that runs the C<gather/react>, but
+the thread that runs the C<emit>. On this thread, there is no handler for the
+control exception thrown by C<take>, causing it to error out.
+
 =head1 X<supply/emit|Control flow,supply emit>
 
 Emits the invocant into the enclosing