Add documentation for 'useDefaultsIfNotSet()' configuration method

Hi,

Spotted that there wasn't any documentation of the useDefaultsIfNotSet() method to ensure default values are used for the children of array configuration nodes.

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | no
| Applies to    | >2.0
| Fixed tickets | 

Cheers,
Craig