documentation of TRACE -after forms should make clear that ARG means value

[Ken Olum]

The manual section on TRACE says "The -after and -all forms [of :condition, :print, and :break] can use sb-debug:arg". I think it should make clear that in the case where the form is being evaluated after the traced function has finished that (sb-debug:arg n) means the nth value being returned, not the nth argument originally supplied.