test: clarify off-loop thread assertions

tests/test_converters.py

@@ -100,7 +100,7 @@ async def test_parse_zip_runs_off_event_loop_thread():
     and DoclingDocument.model_validate — all synchronous and CPU-heavy (full-
     resolution page rasters when generate_page_images is on). It must run off
     the event-loop thread, or it stalls every other worker's coroutine. Capture
-    the thread it runs on and assert it is not the main thread."""
+    the thread it runs on and assert it is not the event-loop thread."""
     import threading
 
     config = AppConfig()

tests/test_pdf_split.py

@@ -163,7 +163,7 @@ async def test_concatenate_runs_off_event_loop_thread(tmp_path, monkeypatch):
     base64 page/picture images — CPU-heavy and proportional to total document
     size. It must run off the event-loop thread so it doesn't stall other
     workers' coroutines. Capture the thread it runs on and assert it is not the
-    main thread."""
+    event-loop thread."""
     import threading
 
     from docling_core.types.doc.document import DoclingDocument