Added jnthn++ doc of Supply.on-close

Author: Jan-Olof Hendig

doc/Type/Supply.pod6

@@ -231,6 +231,26 @@ Creates a supply to which all values seen in the given supply, are emitted
 again.  The given code, executed for its side-effects only, is guaranteed
 to be only executed by one thread at a time.
 
+=head2 method on-close
+
+    method on-close(Supply:D: &on-close --> Supply:D)
+
+Returns a new C<Supply> which will run C<&on-close> whenever a L<Tap|/type/Tap>
+of that C<Supply> is closed. This includes if further operations are chained
+on to the C<Supply>. (for example, C<$supply.on-close(&on-close).map(*.uc)>).
+When using a C<react> or C<supply> block, using the L<CLOSE|/language/phasers#CLOSE>
+phaser is usually a better choice.
+
+my $s = Supplier.new;
+my $tap = $s.Supply.on-close({ say "Tap closed" }).tap(
+    -> $v { say "the value is $v" },
+    done    => { say "Supply is done" },
+    quit    => -> $ex { say "Supply finished with error $ex" },
+);
+
+$s.emit('Perl 6');
+$tap.close;        # OUTPUT: «Tap closed␤»
+
 =head2 method interval
 
     method interval(Supply:U: $interval, $delay = 0 --> Supply:D)