better comment on protocol version getter

Signed-off-by: varun-edachali-dbx <varun.edachali@databricks.com>

Author: varun-edachali-dbx

src/databricks/sql/backend/types.py

@@ -169,7 +169,8 @@ def get_protocol_version(self):
         Get the server protocol version for this session.
 
         Returns:
-            The server protocol version or None if this is not a Thrift session ID
+            The server protocol version or None if it does not exist 
+            It is not expected to exist for SEA sessions.
         """
         return self.properties.get("serverProtocolVersion")