Add a note to has_one association

Mention possibility of multiple associated records.

I'm not sure if the correct way to use `has_one` is to always have a validation that prevents creating multiple associated records, or to never create multiple records in the first place. 

Either way, it seems like `has_one` is easy to do wrong. 

PS, first PR here. Hopefully I didn't break any rules/contribution guidelines/miss existing documentation about this. 

Thanks!