[ci skip] Clarify deletion strategies for collection proxies

For detailed testing of behavior see:
https://gist.github.com/eileencodes/5b0a2fe011dcff6203fe

This shows destroy_all always destroys records and fires callbacks.
It will never use nullify or delete_all

delete_all's behavior varies greatly based on `hm` vs `hm:t` and deletion
strategy.