Warn against using order to determine primacy.

We haven't been explicit that, while the content store preserves
ordering, we shouldn't be using that to express different kinds of
relationships, such as primary sections.
Be explicit about single-item relationships.

For example, a `primary_section` relationship might only make sense with
a single linked item, but for the sake of consistency of representation,
we're sticking with a list of 1 item.

If someone were to implement this incorrectly, this would get caught and
rejected by the model logic anyway, but since people have asked about
this behaviour before, it's worth adding it to the documentation so we
can cite it.
Split up a large paragraph.
Merge pull request #65 from alphagov/primacy

Warn against using order to determine primacy