Improvements of the CollectionType Documentation

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | yes (symfony/symfony#9773)
| Applies to    | 2.5
| Fixed tickets | -

This PR intends to improve the documentation about the side impacts of the require
option, since this option is used also in the default empty_data option.

In addition the new option delete_emtpy according to symfony/symfony#9773 is being
described.

Please verify if me deletions are correct, when I was comparing with the code I found
that those decisions are not based on the compound option, they are based on the
required option.

@bschussek It would be great if you could have a look at it as well. Thanks!