add fail to Control Flow

Author: gfldex

doc/Language/control.pod6

@@ -700,6 +700,16 @@ All these forms may produce a return value the same way C<loop> does.
 
 Same as C<return>, but returns a writable container.
 
+=head1 X<fail|control flow>
+
+Leaves the current routine and returns the provided
+L<Exception|/type/Exception> or C<Str> wrapped inside a
+L<Failure|/type/Failure>.
+
+    sub f { "WELP!" };
+    dd f;
+    # OUTPUT«Failure.new(exception => X::AdHoc.new(payload => "wut?"), backtrace => Backtrace.new)␤»
+
 =head1 X<once|control flow>
 
 A block prefix with C<once> will be executed exactly once, even if placed inside a