Add information about "allow_destroy" requiring an ID. [ci skip]

I just wasted an absurd amount of time trying to figure out why my model wasn't being deleted even though I was setting `_destroy` to true like the instructions said. Making the documentation a little bit clearer so that someone like me doesn't waste their time in future.
I'm not sure we should give a code example with `the following will not work`. The sentence alone is probably enough.