update sdk document (#2374)

### What problem does this PR solve?

_Briefly describe what this PR aims to solve. Include background context
that will help reviewers understand the purpose of the PR._

### Type of change

- [x] New Feature (non-breaking change which adds functionality)

---------

Co-authored-by: Kevin Hu <kevinhu.sh@gmail.com>

sdk/python/ragflow/__init__.py

@@ -5,4 +5,5 @@ __version__ = importlib.metadata.version("ragflow")
 from .ragflow import RAGFlow
 from .modules.dataset import DataSet
 from .modules.assistant import Assistant
-from .modules.session import Session
+from .modules.session import Session
+from .modules.document import Document