Improve grammar and do similar fix for extruder end g-code

Contributes to issue CURA-5906.

resources/definitions/fdmextruder.def.json

@@ -78,7 +78,7 @@
                 "machine_extruder_start_code":
                 {
                     "label": "Extruder Start G-Code",
-                    "description": "Start g-code to execute whenever this extruder is switched to.",
+                    "description": "Start g-code to execute when switching to this extruder.",
                     "type": "str",
                     "default_value": "",
                     "settable_per_mesh": false,
@@ -124,7 +124,7 @@
                 "machine_extruder_end_code":
                 {
                     "label": "Extruder End G-Code",
-                    "description": "End g-code to execute whenever turning the extruder off.",
+                    "description": "End g-code to execute when switching away from this extruder.",
                     "type": "str",
                     "default_value": "",
                     "settable_per_mesh": false,

resources/definitions/fdmprinter.def.json

@@ -2406,7 +2406,7 @@
                 "switch_extruder_retraction_amount":
                 {
                     "label": "Nozzle Switch Retraction Distance",
-                    "description": "The amount of retraction when switching extruder. Set to 0 for no retraction at all. This should generally be the same as the length of the heat zone.",
+                    "description": "The amount of retraction when switching extruders. Set to 0 for no retraction at all. This should generally be the same as the length of the heat zone.",
                     "type": "float",
                     "unit": "mm",
                     "enabled": "retraction_enable",