lnworker.check_mpp_status: add docstring

This explains why stopping_soon behaves sanely with hold-invoice-htlcs.

electrum/lnworker.py

@@ -2269,11 +2269,20 @@ class LNWallet(LNWorker):
             htlc: UpdateAddHtlc,
             expected_msat: int,
     ) -> RecvMPPResolution:
+        """Returns the status of the incoming htlc set the given *htlc* belongs to.
+
+        ACCEPTED simply means the mpp set is complete, and we can proceed with further
+        checks before fulfilling (or failing) the htlcs.
+        In particular, note that hold-invoice-htlcs typically remain in the ACCEPTED state
+        for quite some time -- not in the "WAITING" state (which would refer to the mpp set
+        not yet being complete!).
+        """
         payment_hash = htlc.payment_hash
         payment_key = payment_hash + payment_secret
         self.update_mpp_with_received_htlc(
             payment_key=payment_key, scid=short_channel_id, htlc=htlc, expected_msat=expected_msat)
         mpp_resolution = self.received_mpp_htlcs[payment_key].resolution
+        # if still waiting, calc resolution now:
         if mpp_resolution == RecvMPPResolution.WAITING:
             bundle = self.get_payment_bundle(payment_key)
             if bundle:
@@ -2290,7 +2299,7 @@ class LNWallet(LNWorker):
                 mpp_resolution = RecvMPPResolution.ACCEPTED
             elif time.time() - first_timestamp > self.MPP_EXPIRY:
                 mpp_resolution = RecvMPPResolution.EXPIRED
-
+            # save resolution, if any.
             if mpp_resolution != RecvMPPResolution.WAITING:
                 for pkey in payment_keys:
                     if pkey in self.received_mpp_htlcs: