Showing overmap features you may be missing at world creation screen

Full lists seem unreasonable but showing one random special that can appear if you increase city size by one and one that can if you decrease it sounds good enough.

P.S. overmap specials data can be viewed as a table by using this:
./tools/json_tools/ -f csv --nonestring "" -t overmap_special id city_distance city_sizes occurrences locations > file_will_be_rewritten.csv