docs: document closeThread function

Signed-off-by: Luca Zeuch <[email protected]>

Author: l-zeuch

content/docs/reference/templates/functions.md

@@ -26,6 +26,16 @@ Unless specified otherwise, these functions accept an ID, name, or `nil` for the
 
 Adds a member to an existing thread. Does nothing if either argument is invalid.
 
+### closeThread
+
+```yag
+{{ closeThread <thread> <lock> }}
+```
+
+Closes the given thread.
+
+- `lock`: whether to also lock the thread. Default `false`.
+
 ### createForumPost
 
 ```yag