WIP: Add documentation for relation direactionality

[doenietzomoeilijk]

First draft of the direction documentation.

• Add a section to the ContentTypes / Relationships page
• Change the method signature in the various Twig-method/filter pages
• Add a link from those pages to the new section

Open to suggestions wrt working and amount of detail; it's clear to me but maybe other people need more or different info to make sense of it.