explain that using by_reference only works with add/removeEntityNameSingular in English

[michelepatrassi]

I had to manage a one-to-many relations from ad Event to Images, so after some search I founded this:

• https://symfony.com/doc/current/cookbook/form/form_collections.html

Using by_reference force the framework to use only add/removeEntityNameSingular, so if I have an images property it uses addImage or removeImage...this is well explained in this SO answer:

-http://stackoverflow.com/questions/33358989/symfony2-form-collection-not-calling-addxxx-and-removexxx-even-if-by-reference

The problem is that I use to write code in my native language most of the time, so of course my singular/plural didn't work! I lost a lot of time searching for the root problem elsewhere (es: Doctrine), I think that add this detail to the docs can save time for who will meet the same problem.

[xabbuh]

Sounds like a good idea to improve the cookbook if it is not clear enough. Adding a caution block with a short sentence might be enough.