Document Failure.self

Rakudo impl: rakudo/rakudo@0a100825dd
Tests: Raku/roast@5bdb393ea9

doc/Type/Failure.pod6

@@ -56,6 +56,31 @@ Returns the L<Exception> object that the failure wraps.
     put "$ex.^name(): $ex";
     # OUTPUT: «X::AdHoc: Failed␤»
 
+=head2 method self
+
+Defined as:
+
+    method exception(Failure:D: --> Failure:D)
+
+If the invocant is a L<handled> C<Failure>, returns it as is.
+If not not handled, throws its L<Exception>. Since
+L<Mu> type L«provides C<.self>|/type/Mu#method_self» for every
+class, calling this method is a handy way to explosively
+filter out Failures:
+
+    my $num1 = '♥'.Int;
+    # $num1 now contains a Failure object, which may not be desirable
+
+    my $num2 = '♥'.Int.self;
+    # .self method call on Failure causes an exception to be thrown
+
+    my $num3 = '42'.Int.self;
+    # Int type has a .self method, so here $num3 has `42` in it
+    
+    (my $stuff = '♥'.Int).so;
+    say $stuff.self; # OUTPUT: «(HANDLED) Cannot convert string to number…»
+    # Here, Failure is handled, so .self just returns it as is
+
 =head2 method Bool
 
 Defined as: