README: Improvements to UI event object docs (#444)

Closes #316 .

Signed-off-by: Richard Koshak <rkoshak@users.noreply.github.com>

Author: rkoshak

README.md

@@ -138,10 +138,11 @@ See [openhab-js](https://openhab.github.io/openhab-js) for a complete list of fu
 ### UI Event Object
 
 **NOTE**: Note that `event` object is different in UI based rules and file based rules! This section is only valid for UI based rules. If you use file based rules, refer to [file based rules event object documentation](#event-object).
-Note that `event` object is only available when the UI based rule was triggered by an event and is not manually run!
-Trying to access `event` on manual run does not work (and will lead to an error), use `this.event` instead (will be `undefined` in case of manual run).
+Note that `event` object is only available when the UI based rule was triggered by an event and is not called from another rule!
+Trying to access `event` in this case does not work and will lead to an error. Use `this.event` instead (will be `undefined` when it does not exist).
 
 When you use "Item event" as trigger (i.e. "[item] received a command", "[item] was updated", "[item] changed"), there is additional context available for the action in a variable called `event`.
+When a rule is triggered, there is additional context available for the action in a variable called `event`.
 
 This table gives an overview over the `event` object for most common trigger types:
 
@@ -152,8 +153,10 @@ This table gives an overview over the `event` object for most common trigger typ
 | `itemCommand`  | sub-class of [org.openhab.core.types.Command](https://www.openhab.org/javadoc/latest/org/openhab/core/types/command) | `[item] received a command`            | Command that triggered event                                                                                  | `receivedCommand`      |
 | `itemName`     | string                                                                                                               | all                                    | Name of Item that triggered event                                                                             | `triggeringItem.name`  |
 | `type`         | string                                                                                                               | all                                    | Type of event that triggered event (`"ItemStateEvent"`, `"ItemStateChangedEvent"`, `"ItemCommandEvent"`, ...) | N/A                    |
+| `event`        | string                                                                                                               | channel based triggeres                | Event data published by the triggering channel.                                                               | `receivedEvent`        |
+| `payload`      | JSON formatted string                                                                                                | all                                    | Any additional information provided by the trigger not already exposed. "{}" there is none.             | N/A                    |
 
-Note that in UI based rules `event.itemState`, `event.oldItemState`, and `event.itemCommand` are Java types (not JavaScript), and care must be taken when comparing these with JavaScript types:
+Note that in UI based rules `event`, and therefore everything carried by `event` are Java types (not JavaScript). Care must be taken when comparing these with JavaScript types:
 
 ```javascript
 var { ON } = require("@runtime")