Clarify the use of the armrest key

Use a combo instead of a checkbox in order to provide better information
about the values and their meaning.

#1226

data/fields/armrest.json

@@ -1,5 +1,13 @@
 {
     "key": "armrest",
-    "type": "check",
-    "label": "Armrest(s)"
+    "type": "combo",
+    "label": "Armrests",
+    "placeholder": "No armrests, One or more armrests...",
+    "strings": {
+        "options": {
+            "no": "No armrests",
+            "yes": "One or more armrests"
+        }
+    },
+    "customValues": false
 }