Improve documentation for feature-state expression in style-spec

src/style-spec/reference/v8.json

@@ -2606,7 +2606,7 @@
         }
       },
       "feature-state": {
-        "doc": "Retrieves a property value from the current feature's state. Returns null if the requested property is not present on the feature's state object.",
+        "doc": "Retrieves a property value from the current feature's state. Returns null if the requested property is not present on the feature's state. A feature's state is not part of the GeoJSON or vector tile data, and must be set programmatically on each feature. Note that [\"feature-state\"] can only be used with paint properties that support data-driven styling.",
         "group": "Feature data",
         "sdk-support": {
           "basic functionality": {