adding docs and an example for nextcallee

doc/Language/functions.pod6

@@ -727,6 +727,20 @@ class LoggedVersion is Version {
 say LoggedVersion.new('1.0.2');
 =end code
 
+If you need to make multiple calls to the wrapped code or to gain a reference to
+it, such as for performing introspection on the it, you can use C<nextcallee>.
+
+=begin code
+sub power-it($x) { $x * $x }
+sub run-it-again-and-again($x) {
+    my &again = nextcallee;
+    again again $x;
+}
+
+&power-it.wrap(&run-it-again-and-again);
+say power-it(5);    # 625
+=end code
+
 =head1 Coercion Types
 
 Coercion types can help you to have a specific type inside a routine, but