This is only shown when viewing one edit at a time, and does not point
to docs for the specific edit either, but to a generic page.
It should be replaced by a documentation link that is visible
from the edit header on any edit view.

This adds a link to the doc page for the specific edit type in question
to the header of every edit, be it on an edit list or
on the edit page itself.
This replaces the generic "Edit Types" link on the sidebar
(not shown on edit lists) that was removed in the previous commit.

This was probably copied from Create or other place where the structure
was different, but here {link} is inside {relationship}.

I'm using this outside the relationship editor in the next commit,
so it seems cleaner to move it here.

This adds a link to the doc page(s) for the relationship type(s)
involved in a relationship edit (add, edit or remove),
and lists the type descriptions for a basic overview.
It seems very useful to have direct links to the documentation
(often including specific guidelines)
directly in the place people need to vote on the changes from.

We pass an array to allow multiple types in EditRelationship;
we pass the rels through a Set to remove duplicates
when EditRelationship does not change the rel type.