hugo: explain how to upgrade roughness from v21 to v22; what about H=0?

This issue is a remnant of #1165 (closed).

• As the upgrade-21-to-22 script cannot cope with roughness, the online reference should explain in full detail how to do the upgrade manually. --- !3063 (merged)
• The old default, hurst=0, is no longer allowed in v22. What computation was actually done for hurst=0 in v21? --- Answer: for hurst=0, there was no scattering contribution from the interface. With !3034 (merged), H=0, H=1, corrLength=0 are again allowed.