Finish the rest of the event documentation (#683)

doc/events/timer.md

@@ -0,0 +1,21 @@
+---
+module: [kind=event] timer
+see: os.startTimer To start a timer.
+---
+
+The @{timer} event is fired when a timer started with @{os.startTimer} completes.
+
+## Return Values
+1. @{string}: The event name.
+2. @{number}: The ID of the timer that finished.
+
+## Example
+Starts a timer and then prints its ID:
+```lua
+local timerID = os.startTimer(2)
+local event, id
+repeat
+    event, id = os.pullEvent("timer")
+until id == timerID
+print("Timer with ID " .. id .. " was fired")
+```