Add more details for why we set num-threads to 1. The added text has been copied from an Arch Linux page.

Signed-off-by: DL6ER <dl6er@dl6er.de>

docs/guides/unbound.md

@@ -101,7 +101,7 @@ server:
     # This only applies to domains that have been frequently queried
     prefetch: yes
 
-    # One thread should be sufficient, can be increased on beefy machines
+    # One thread should be sufficient, can be increased on beefy machines. In reality for most users running on small networks or on a single machine it should be unnecessary to seek performance enhancement by increasing num-threads above 1.
     num-threads: 1
 
     # Ensure kernel buffer is large enough to not lose messages in traffic spikes