Document example usage of object_id_name
I found the text and admonition confusing, even in context. I'll pass for now but will think about whether some improvement to the docs is needed for this situation.
In combination with `column_name` defined, an AttributeError is raised otherwise.
This behavior was confusing me at first, until I found #1302. The docs (apart from the ForeignKeyField API) don't mention object_id_name