Improve explanation/text of primary vs foreign keys

[lwjohnst86]

Me and @joelostblom were having a discussion and it prompted me to think that we should simplify how we describe primary/foreign keys (considering it isn't a very intuitive/common way of thinking about data for the average reader).

So maybe instead of:

- Primary key(s):

It could be:

- Unique identifier(s) ("primary keys"):

And for foreign keys:

- Identifier(s) linking to other resources ("foreign keys"):