UiTargetCamera doc comment correction

[ickshonpe]

Objective

As pointed out in #18177 this line in the doc comment for UiTargetCamera:

/// Optional if there is only one camera in the world. Required otherwise.

is incorrect. UiTargetCamera component is only needed when you want to display UI nodes using a camera other than the default camera.

Solution

Change it to:

/// Root node's without an explicit [`UiTargetCamera`] will be rendered to the default UI camera,
/// which is either a single camera with the [`IsDefaultUiCamera`] marker component or the highest
/// order camera targeting the primary window.