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

• https://github.com/silverstripe/silverstripe-framework/issues/9612

Pull request checklist

• [x] The target branch is correct
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• [x] All commits are relevant to the purpose of the PR (e.g. no TODO comments, unrelated rewording/restructuring, or arbitrary changes)
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• [x] The commit messages follow our commit message guidelines
• [x] The PR follows our contribution guidelines
• [x] The changes follow our writing style guide
• [x] Code examples follow our coding conventions
• [x] CI is green

[edwilde]

Thanks for the speedy turnaround @GuySartorelli 🚀