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.

resources/definitions/fdmprinter.def.json

@@ -1890,7 +1890,7 @@
                 "infill_pattern":
                 {
                     "label": "Infill Pattern",
-                    "description": "The pattern of the infill material of the print. The line and zig zag infill swap direction on alternate layers, reducing material cost. The grid, triangle, tri-hexagon, cubic, octet, quarter cubic, cross and concentric patterns are fully printed every layer. Gyroid, cubic, quarter cubic and octet infill change with every layer to provide a more equal distribution of strength over each direction. Lightning infill tries to minimize the infill, by only supporting the (internal) roofs of the object. As such, the infill percentage is only 'valid' one layer below whatever it needs to support of the model.",
+                    "description": "The pattern of the infill material of the print. The line and zig zag infill swap direction on alternate layers, reducing material cost. The grid, triangle, tri-hexagon, cubic, octet, quarter cubic, cross and concentric patterns are fully printed every layer. Gyroid, cubic, quarter cubic and octet infill change with every layer to provide a more equal distribution of strength over each direction. Lightning infill tries to minimize the infill, by only supporting the ceiling of the object.",
                     "type": "enum",
                     "options":
                     {