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 clear so
that someone like me doesn't waste their time in future.
Merge pull request #18145 from georgemillo/patch-2

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