Add some note about missing dovecot alerts

nix/systems/x86_64-linux/monitoring.kaareskovgaard.net/alerts/dovecot.yaml

@@ -1,5 +1,9 @@
 - name: Dovecot
   rules:
+    # This actually doesn't work as there's no dovecot_up exported.
+    # This should probably be dynamically created as the job_up.nix file does it,
+    # for every server that uses dovecot. It should be possiblet to query dovecot_build_info
+    # with absent_over_time somehow.
     - alert: DovecotDown
       expr: >
         dovecot_up{job="dovecot"} == 0