Expand iMOD5 backwards compatibility documentation on HFBs

[JoerivanEngelen]

iMOD5 lets you define resistance in two manners in the projectfile:

1. negative resistance: Barrier resistance overrides resistance between model cells
2. positive resistance: Barrier resistance is added to the background resistance of the aquifer

MODFLOW6 lets you define resistances via a hydraulic characteristic:

• a. negative hydraulic characteristic: Value is multiplied with conductance inbetween model cells
• b. positive hydraulic characteristic: Barrier resistance is added to the background resistance of the aquifer

I've noticed some differences between iMOD5 and iMOD Python along a fault line, which in the end pertained to a bug in iMOD5. iMOD5 erroneously adds a background resistance to the HFB resistance, which MODFLOW6 does as well. This isn't such an issue in permeable layers, but gets you twice the resistance in clay layers. I think it is good to add this in the "Known issues" section in the documentation (despite it being an iMOD5 bug).

Furthermore, it is good to add that iMOD Python currently doesn't support iMOD5's option 1.