Fix documentation (#630)

* Fix missing documentation

* Add test

* Add test

* Format

* Reformat

tests/test_documentation.py

@@ -0,0 +1,29 @@
+import ast
+import inspect
+
+
+def test_documentation():
+    from .output_betterproto.documentation import (
+        Enum,
+        ServiceBase,
+        ServiceStub,
+        Test,
+    )
+
+    assert Test.__doc__ == "Documentation of message"
+
+    source = inspect.getsource(Test)
+    tree = ast.parse(source)
+    assert tree.body[0].body[2].value.value == "Documentation of field"
+
+    assert Enum.__doc__ == "Documentation of enum"
+
+    source = inspect.getsource(Enum)
+    tree = ast.parse(source)
+    assert tree.body[0].body[2].value.value == "Documentation of variant"
+
+    assert ServiceBase.__doc__ == "Documentation of service"
+    assert ServiceBase.get.__doc__ == "Documentation of method"
+
+    assert ServiceStub.__doc__ == "Documentation of service"
+    assert ServiceStub.get.__doc__ == "Documentation of method"