docs(ocr): clarify entrypoint comment and add manual run hint for skipped test

- entrypoint.sh: replace "cross-job ground-truth leakage" with plain
  "Remove stale partial downloads left by a previous docker-kill"
- test_tmpdir_is_inside_persistent_cache_volume: add docker exec command
  so future developers know how to run this deployment-contract test

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

ocr-service/test_tmpdir.py

@@ -90,6 +90,7 @@ def test_tmpdir_is_inside_persistent_cache_volume():
 
     Catches accidental reversion to /tmp or any tmpfs-backed path.
     Runs only inside the OCR Docker container where TMPDIR=/app/cache/.tmp.
+    To run manually: docker exec archiv-ocr python -m pytest test_tmpdir.py::test_tmpdir_is_inside_persistent_cache_volume -v
     See ADR-021.
     """
     tmpdir = os.environ["TMPDIR"]