Elaborate on onEvent documentation

docs/objects/Object/onEvent.md

@@ -14,7 +14,7 @@ You can find a full list here: [CC:Tweaked](https://tweaked.cc/) (on the left si
 
 ### Usage
 
-* Add an onEvent event to your frame:
+Add an onEvent event to your frame:
 
 ```lua
 local basalt = require("basalt")
@@ -26,3 +26,22 @@ main:onEvent(function(event, side, channel, replyChannel, message, distance)
     end
 end)
 ```
+
+The parameters passed to this function are the same as those returned by `os.pullEvent()`. See [here](https://computercraft.info/wiki/Os.pullEvent) for more info.
+
+Alternatively, you can add an onEvent event to an XML layout:
+
+```xml
+<onEvent>
+    local eventType = event[3]
+    local message = event[7]
+    if (eventType == "modem_message") then
+        basalt.debug("Message received: " .. tostring(message))
+    end
+</onEvent>
+```
+
+In this case, the event table indices correspond to:
+* 1: The Object where this event is being handled
+* 2: A string representing the Basalt event type. This will always just be the string `"other_event"`
+* 3 onwards: the `os.pullEvent()` parameters