clarify comp / assoc w.r.t. deep ops

[David-Kunz]

What's still missing / not clear:

• The target rows of a composition cannot exist without the parent (they're deleted automatically).
• It's disallowed to have multiple compositions pointing to the same rows as they take ownership over them.

Please let me know what you think!

[renejeglinsky]

@David-Kunz @beckermarc @danjoa How to proceed here?