routing: rename method and add err check when launch shard

A followup commit for PR#5332. In this commit we add more docs, rename
function updatePaymentState to fetchePaymentState, and add back the
check for channeldb.ErrPaymentTerminal after we launch shard.

routing/payment_lifecycle_test.go

@@ -1083,7 +1083,7 @@ func TestUpdatePaymentState(t *testing.T) {
 			}
 
 			// Call the method that updates the payment state.
-			_, state, err := pl.updatePaymentState()
+			_, state, err := pl.fetchPaymentState()
 
 			// Assert that the mock method is called as
 			// intended.