Merge pull request #899 from MihailRis/app-new-instance

engine sub-instance spawn function

doc/en/scripting/builtins/libapp.md

@@ -180,14 +180,34 @@ app.get_content_sources() -> table<string>
 
 Returns a list of content sources (paths), in descending priority order.
 
-``lua
+```lua
 app.set_content_sources(sources: table<string>)
 ```
 
 Sets a list of content sources (paths). Specified in descending priority order.
 
-``lua
+```lua
 app.reset_content_sources()
 ```
 
 Resets content sources.
+
+## Sub-instances
+
+```lua
+-- Creates a headless engine instance with the current project and the specified application script.
+-- Returns the instance ID. The number of active sub-instances is currently limited to one.
+app.start_background_instance(
+    -- script file
+    app_script: string,
+    -- log file
+    output_file: string | nil
+) -> int
+
+-- Checks if the engine sub-instance is alive.
+app.is_instance_alive(handle: int) -> boolean
+
+-- Stops the engine sub-instance.
+-- Returns true if the instance was alive at the time of the call.
+app.terminate(handle: int) -> boolean
+```