Fixes #1773 and adds clarification

doc/Type/Exception.pod6

@@ -43,15 +43,13 @@ an exception itself.
 
 Defined as:
 
-    method backtrace(Exception:D: --> Backtrace:D)
+    method backtrace(Exception:D:)
 
-Returns the backtrace associated with the exception. Only makes sense
-on exceptions that have been thrown at least once.
+Returns the backtrace associated with the exception or an empty string if there
+is none. Only makes sense on exceptions that have been thrown at least once.
 
     try die "Something bad happened";
-    if ($!) {
-        print $!.backtrace;
-    }
+    with $! { .backtrace.print ; }
 
 =head2 method throw
 

doc/Type/X/AdHoc.pod6

@@ -10,9 +10,9 @@ class X::AdHoc is Exception { }
 C<X::AdHoc> is the type into which objects are wrapped if they are
 thrown as exceptions, but don't inherit from L<Exception|/type/Exception>.
 
-Its benefit over returning non-C<Exception> objects is that it gives
-access to all the methods from class L<Exception|/type/Exception>, like C<backtrace>
-and C<rethrow>.
+Its benefit over returning non-C<Exception> objects is that it gives access to
+all the methods from class L<Exception|/type/Exception>, like C<backtrace> and
+C<rethrow>.
 
 You can obtain the original object with the C<payload> method.
 
@@ -22,13 +22,14 @@ You can obtain the original object with the C<payload> method.
     print "Got HTTP code ",
         $!.payload[0],          # 404
         " and backtrace ",
-        $!.backtrace;
-
-Note that young code will often be prototyped using C<X::AdHoc> and
-then later be revised to use more specific subtypes of C<Exception>.
-As such it is usually best not to explicitly rely on receiving an
-C<X::AdHoc> – in many cases using the string returned by the
-C<.message> method, which all C<Exception>s must have, is preferable.
+        $!.backtrace.Str;
+
+Note that young code will often be prototyped using C<X::AdHoc> and then later
+be revised to use more specific subtypes of C<Exception>. As such it is usually
+best not to explicitly rely on receiving an C<X::AdHoc> – in many cases using
+the string returned by the C<.message> method, which all C<Exception>s must
+have, is preferable. Please note that we need to explicitly call C<.Str> to
+stringify the backtrace correctly.
 
 =head1 Methods