libpasswdqc.3: provide a top-level definition of passwdqc_params_t

[ldv-alt]

This description should make clear the relationship between passwdqc_params_t and passwdqc_params_qc_t types.

[solardiz]

Not ideal, but something like this was in fact needed - otherwise it's unclear what to pass into some functions (except from the example).