Clarified Collection.create documentation

## Description
In the current documentation for `Collection.create`, it is stated that `false` will be returned if model validation fails.

I've clarified that you must pass the `{validation: true}` option in order for `create` to return false if model validation fails.

Currently, unless a developer dives fairly deep into the code, it isn't clear from the documentation why `create` will return a model event if model validation fails.

## Code References
Here the `Collection._prepareModel` method determines wether to return an instance of the model or `false`, depending on the result of `Model._validate`.

https://github.com/hiwaylon/backbone/blob/master/backbone.js#L871

Here the `Model._validate` method checks the `validate` option and the existence of the `validate` method on the model. Only if both are `true` will it run validation.

https://github.com/hiwaylon/backbone/blob/master/backbone.js#L545