Improve description of steps per mm (E) setting

Technically, moving the feeder wheel by 1mm doesn't guarantee that the filament gets extruded by 1mm, since there might be slip.

Reported in issue #9924.

resources/definitions/fdmprinter.def.json

@@ -704,7 +704,7 @@
                 "machine_steps_per_mm_e":
                 {
                     "label": "Steps per Millimeter (E)",
-                    "description": "How many steps of the stepper motors will result in one millimeter of extrusion.",
+                    "description": "How many steps of the stepper motors will result in moving the feeder wheel by one millimeter around its circumference.",
                     "type": "float",
                     "default_value": 1600,
                     "minimum_value": "0.0000001",