[ci skip] Documented delete_all dependent parameter for rdoc

Yep I see your point. I'm working through `collection_proxy.rb` to improve the documentation but I thought it made sense to fix up the existing documentation for rdoc beforehand so that the coverage report gives a clearer picture of what needs to be documented.

Would you advise leaving this as `<tt>:dependent</tt>` or changing to `+dependent+`?