signal: clarify the log message about systemd

The log message was confusing because it was emitted even when running
within systemd if the service type was **not** `notify`. This clarifies
the message by mentioning the edge case.

Being able to distinguish the two cases would be nicer but there doesn't
seem to be a reasonably simple, obvious, reliable way to do it.
Complicated solutions are most likely not worth it for a simple log
message.

docs/release-notes/release-notes-0.14.2.md

@@ -42,6 +42,9 @@
 * [Ensure that the min relay fee is always clamped by our fee
   floor](https://github.com/lightningnetwork/lnd/pull/6076)
 
+* [Clarify log message about not running within
+  systemd](https://github.com/lightningnetwork/lnd/pull/6096)
+
 ## RPC Server
 
 * [ChanStatusFlags is now