Clarify description of Lightning infill

Two changes have been made:
- Use more consistent terminology for the top side of the model (which could be skin or walls). This is corrected from feedback by our translators.
- Don't add disclaimers about the infill density to this already way-too-long setting description. If anything, a disclaimer should be added to the infill density description that Gradual Infill and some infill patterns might not reach the desired density everywhere. But really, I think it's more in the domain of user documentation like the Ultimaker website or the Settings Guide.

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