docs: update callback doc

Author: makspll

docs/src/ScriptingReference/core-callbacks.md

@@ -10,6 +10,8 @@ The two core callbacks are:
 
 This will be called right after a script has been loaded or reloaded. This is a good place to initialize your script. You should avoid placing a lot of logic into the global body of your script, and instead put it into this callback. Otherwise errors in the initialization will fail the loading of the script.
 
+This callback will not have access to the `entity` variable, as when the script is being loaded it's not attached to an entity yet.
+
 ```lua
 print("you can also use this space, but it's not recommended")
 function on_script_loaded()
@@ -21,8 +23,10 @@ end
 
 This will be called right before a script is unloaded. This is a good place to clean up any resources that your script has allocated. Note this is not called when a script is reloaded, only when it is being removed from the system.
 
+This callback will not have access to the `entity` variable, as when the script is being unloaded it might not be attached to an entity.
+
 ```lua
 function on_script_unloaded()
     print("Goodbye world")
 end
-```
+```