Update Documentation to Include `foreign_key`

[jvanderen1]

In the example for Relationship with user defined foreign key, there includes a foreign_key field in the JSON.

However, in the official schema definition documentation, there is no foreign_key field listed. It would be nice to have this available.

Some outstanding questions I have:

• Does foreign_key work with through_tables at all?
• Can you specify other columns to join with? For example, if my composite key in Table C is comprised by fields in both Table A and Table B, can I specify which column(s) from specific table(s) to join on?