[PD] Tiny register info field cleanup for mooncake backend (#22016)

python/sglang/srt/disaggregation/mooncake/conn.py

@@ -81,6 +81,7 @@ class TransferInfo:
     dst_state_indices: List[int]
     required_dst_info_num: int
     is_dummy: bool
+    # Note: always put the optional staging field at the final (it will be set through 'STAGING_RSP' pkg when needed)
     staging: Optional[StagingTransferInfo] = None
 
     @classmethod
@@ -129,6 +130,7 @@ class KVArgsRegisterInfo:
     dst_state_dim_per_tensor: list[int]
     # HiSparse: decode host pool stores KV at token granularity
     enable_hisparse: bool = False
+    # Note: always put the staging field at the final (since the staging field is optional and contains multiple inputs)
     staging: Optional[StagingRegisterInfo] = None
 
     @classmethod
@@ -157,6 +159,7 @@ class KVArgsRegisterInfo:
             enable_hisparse=(
                 msg[12].decode("ascii") == "1" if len(msg) > 12 else False
             ),
+            # Note: always put the staging field at the final
             staging=StagingRegisterInfo.from_zmq_fields(msg, 13),
         )