Update furniture & terrain docs

[Milopetilo]

Summary

None

Purpose of change

Some fields weren't said to be optional even though they are, move_cost_mod was also not explained properly in furniture.

Describe the solution

Add optional to fields that are optional and add move_cost_mod to fields shared between furniture and terrain.

Describe alternatives you've considered

Have the same description twice

Testing

Looks alright locally

Additional context