Finish the rest of the event documentation (#683)

doc/events/task_complete.md

@@ -0,0 +1,28 @@
+---
+module: [kind=event] task_complete
+see: commands.execAsync To run a command which fires a task_complete event.
+---
+
+The @{task_complete} event is fired when an asynchronous task completes. This is usually handled inside the function call that queued the task; however, functions such as @{commands.execAsync} return immediately so the user can wait for completion.
+
+## Return Values
+1. @{string}: The event name.
+2. @{number}: The ID of the task that completed.
+3. @{boolean}: Whether the command succeeded.
+4. @{string}: If the command failed, an error message explaining the failure. (This is not present if the command succeeded.)
+...: Any parameters returned from the command.
+
+## Example
+Prints the results of an asynchronous command:
+```lua
+local taskID = commands.execAsync("say Hello")
+local event
+repeat
+    event = {os.pullEvent("task_complete")}
+until event[2] == taskID
+if event[3] == true then
+  print("Task " .. event[2] .. " succeeded:", table.unpack(event, 4))
+else
+  print("Task " .. event[2] .. " failed: " .. event[4])
+end
+```