Change the documentation around link types

The previous documentation said that the keys represented the type of the linked item (`organisations` or `topics`, for example); it's more accurate to say that it's the type of the relationship itself.

For example, the `related` link type doesn't care about the type of the thing it's linking to. As anotehr example, we could feasibly add `next` or `previous` links for a series of items (multi-part guides, say), or `up` links to a parent section.