Update Overmap documentation to include missing field.

[QuillInkwell]

Summary

Bugfixes "Update Overmap Documentation to include field missing from the city specials section."

Purpose of change

Awhile back #72815 made it possible to use the city_sizes field in city buildings. But it isn't shown in the documentation that you can use that field.

I was just thinking about Aftershock maps (as I do) and I thought I could really use that feature about now. So I checked the doc and no luck city buildings don't have that field. So I check the issues, no one's asked for it. So I checked the closed issues and there it is, already in the game.

Describe the solution

Add the field to the documentation.

Describe alternatives you've considered

Leave the docs as they are.

Testing

Checked the doc on my fork. Field is there. Feels good man.

Additional context