DOC Update cascade_deletes information for has_many relationships

[edwilde]

Description

Given that this issue has been open for some time, we should update the documentation which is giving incorrect information to developers.

I have added an example of how best to manage cascade_deletes between many_many related objects.

To avoid adding confusion, I decided against adding the alternative solution using onBeforeDelete() and removeAll(). However this can also be added if you think it is needed, or better?

Issues

• cascade_delete deletes end of relationship rather than join record silverstripe-framework#9612

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[GuySartorelli]

Thanks for making those changes - just one small addition and then I'll be happy.

[GuySartorelli]

Thanks for doing this, it's a great clarification to have.

I'll merge once CI goes green (all our runners are being used right now so it might be a bit)

[edwilde]

Thanks for the speedy turnaround @GuySartorelli 🚀