Consider improving the self relations docs

[andrew-walford-prisma]

From the user feedback in this call log: https://prisma.productboard.com/insights/notes/all-notes/notes/27266684

"Docs for self relations

• Need to use two fields for relation but why and how does it work? Would be great to improve this. Is this mandatory? Why is the relation exactly the same?
• Misguiding in docs: why would you want to pick explicit over implicit many-to-many"

[janpio]

(second bullet is not about self relation)