Document the icon and colour names a forecast can carry

docs/reference/payload-types.md

@@ -174,6 +174,28 @@ carry one of them literally is asking for a value this engine reads as a name.
 
 Enforced on the server as well as in the editor.
 
+## Icon and colour names
+
+A forecast is a `list` of records, one per column, each shaped
+`{label, icon, value}` with an optional `color`. The flow decides what a
+column is called and which glyph it gets; the widget only draws it.
+
+```json
+[{"label": "Today", "icon": "cloud-rain", "color": "primary", "value": "14°"}]
+```
+
+`icon` and `color` are names from a fixed vocabulary, the same one the icon
+widget's picker offers:
+
+| | Names |
+|---|---|
+| Icons | `sun`, `moon`, `cloudy`, `cloud-rain`, `cloud-drizzle`, `cloud-snow`, `cloud-lightning`, `cloud-fog`, `wind`, `umbrella`, `snowflake`, `droplets`, `thermometer`, `thermometer-sun`, `flame`, `fan`, `sun-snow`, `door-open`, `house`, `bed`, `lightbulb`, `plug`, `music`, `volume-2`, `volume-x`, `zap`, `battery-charging`, `arrow-up`, `arrow-down`, `triangle-alert`, `check`, `circle-check` |
+| Colours | `default`, `muted`, `primary`, `success`, `danger` |
+
+An empty icon draws nothing. An unknown one draws a placeholder glyph and an
+unknown colour draws as `default`; either warns once in the browser console,
+listing the names it expected.
+
 ## Type failures
 
 A value that does not match its port's declaration raises on the node that