Anybody know where to find what the acceptable arguments to --trace on the command line are, and what each means?

So far, I've tried:

--trace � � � � � � � � Lots of output
--trace=0 � � � � � � � No output
--trace=1 � � � � � � � Looks maybe the same as --trace with no argument
--trace=2 � � � � � � � No output
--trace=3 � � � � � � � A whaling big lot of very interesting output

Slightly better descriptions of these would be nice. :-)