Make Run in JIT wrapper note only last value is mutated outparam

The other parameters passed to `Run()` that the JIT wrapper creates -- except for the last -- are effectively const, but we need to have the last parameter be an effective "outparam" since the idea is to avoid allocating things to provide back to the caller (that's why they're the packed types). See relatedly https://github.com/google/xls/issues/416#issuecomment-857290800