Improve the UserServicesFactory Form default handling

                   
|Q            |A  |
|---          |---|
|Bug Fix?     |y  |
|New Feature? |y  |
|BC Breaks?   |n  |
|Deprecations?|n  |
|Tests Pass?  |y  |
|Fixed Tickets|   |
                   
Related to GH-55 GH-56

This improves the UserServicesFactory Form default handling

* Detect if a custom class is used with the default fos form-type
* Change 'only' the name when it uses the fos_user prefix

This removes the requirement to set the class, type and the name.
And fixes an edge case where only the name uses the fos_user prefix

And updates the documentation, which was missing the name in the example.