routing+docs: make sure non-MPP cannot use skipTempErr

docs/release-notes/release-notes-0.18.0.md

@@ -91,6 +91,12 @@
   hash](https://github.com/lightningnetwork/lnd/pull/8106) to the
   signer.SignMessage/signer.VerifyMessage RPCs.
 
+* `sendtoroute` will return an error when it's called using the flag
+  `--skip_temp_err` on a payment that's not a MPP. This is needed as a temp
+  error is defined as a routing error found in one of a MPP's HTLC attempts.
+  If, however, there's only one HTLC attempt, when it's failed, this payment is
+  considered failed, thus there's no such thing as temp error for a non-MPP.
+
 ## lncli Updates
 ## Code Health