Improvements of the CollectionType Documentation

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/symfons#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!