Improve NC callback docs

- Fix incorrect constraint syntax
- Link to fuller sig constraint docs

Author: zoffixznet

doc/Language/nativecall.pod6

@@ -649,11 +649,11 @@ say $esponja;
 NativeCall also supports native functions that take functions as arguments.  One
 example of this is using function pointers as callbacks in an event-driven
 system.  When binding these functions via NativeCall, one need only provide the
-equivalent signature as a constraint on the code parameter:
+equivalent signature as L<a constraint on the code parameter|/type/Signature.html#Constraining_signatures_of_Callables>:
 
     use NativeCall;
     # void SetCallback(int (*callback)(const char *))
-    my sub SetCallback(&callback (Str --> int32)) is native('mylib') { * }
+    my sub SetCallback(&callback:(Str --> int32)) is native('mylib') { * }
 
 Note: the native code is responsible for memory management of values passed to
 Perl 6 callbacks this way. In other words, NativeCall will not free() strings passed