Add style-spec note on in type system hint mapbox#9373 (mapbox#10833)

src/style-spec/reference/v8.json

@@ -2698,7 +2698,7 @@
         }
       },
       "in": {
-        "doc": "Determines whether an item exists in an array or a substring exists in a string.",
+        "doc": "Determines whether an item exists in an array or a substring exists in a string. In the specific case when the second and third arguments are string literals, you must wrap at least one of them in a [`literal`](#types-literal) expression to hint correct interpretation to the [type system](#type-system).",
         "group": "Lookup",
         "sdk-support": {
           "basic functionality": {