[2.0.52] Update map-gen-settings.example.json

[Gweneph]

From my testing it seems like there's at least three changes that are not documented in map-gen-settings.exmaple.json and they may be more.

1. The seed field no longer seems to apply. Setting to the seed to a number and repeatedly calling --create with the settings keeps generating different maps with different seeds. I'd prefer if this worked, but --map-gen-seed still works; so it just makes life harder by splitting settings across different parameters. In any case, the documentation doesn't match existing behavior.

2. cliff_settings>richness says it will produce solid cliffs at 10, but it seems that above 6 it disables cliffs entirely.

3. autoplace_controls>nauvis_cliffs seems to modify those cliff_settings. Frequency seems to divide cliff_elevation_interval and size seems to multiply richness such that the product seems to control how long cliffs get (but I haven't been able to get them to be totally solid).