Update handling-relations.md with an example of ReferenceArrayInput

[loeyan]

It took me plenty of hours to get nested objects with referenceArrayInput running. By a lucky hit i found the fix for it! (https://html.developreference.com/article/11019200/How+to+reuse+subresource+data+for+referenced+inputs+in+React-admin%3F)

Also i struggeled with the referenceInput. Please at those special cases for nested. Feel free to edit :-)