$trace docs are incomplete

[petdance]

The POD says

The C<$trace> global variable can be used to control the verbosity of the
high level functions. Level 0 guarantees silence, level 1 (the default)
only emits error messages.

However, looking through the code there are checks for $trace>1, $trace>2, $trace>3 and $trace==3, so there must be other levels. They aren't described.