Added documentation section on Lua garbage collection.

Docs/Reference.dox

@@ -609,6 +609,30 @@ As seen above from the event handling examples, VariantMap handling has some dif
 
 For the rest of the functions and classes, see the generated \ref LuaScriptAPI "Lua script API reference". Also, look at the Lua counterparts of the sample applications in the Bin/Data/LuaScripts directory and compare them to the C++ and AngelScript versions to familiarize yourself with how things are done on the Lua side.
 
+\section LuaScripting_Allocation Object allocation & Lua garbage collection
+
+There are two ways to allocate a C++ object in Lua scripting, which behave differently with respect to Lua's automatic garbage collection:
+
+1) Call object's contructor:
+\code
+local context = GetContext()
+local scene = Scene(context)
+\endcode
+tolua++ will register this C++ object with garbage collection, and Lua will collect it eventually. Do not use this form if you will add the
+object to an object hierarchy that is kept alive on the C++ side with SharedPtr's, for example child scene nodes or %UI child elements.
+Otherwise the object will be double-deleted, resulting in a crash.
+
+
+2) Call the new function:
+\code
+local context = GetContext()
+local text = Text:new(context)
+\endcode
+When using this form the object will not collected by Lua, so it is safe to pass into C++ object hierarchies.
+Otherwise, to prevent memory leaks it needs to be deleted manually by calling the delete function on it:
+\code
+text:delete()
+\endcode
 
 \page Rendering Rendering