Update CollectionField.rst file for better documentation (adding extra considerations for its implementation)

[FerJoTello]

Adding some extra considerations in the docs to use CollectionField class, since this class uses "add" & "remove" methods related to the Collection property from the entity that owns the collection in order to add and delete binded values; there is no documentation that explains that Collections must implement these methods and it is not intuitive so it would be a great help for anyone who is implementing this class for the first time (as it was in my case, after finding the solution 4 hours later).

References: https://stackoverflow.com/questions/48304399/q-symfony4-easyadmin-onetomany-not-saving-in-db