fix(snmp): document + restart after silent tsys9 drive poller wedge [#344]

12h stale despite healthy OID; registry cycle can't revive legacy
snmp pollers - core restart required.
https://projects.knownelement.com/issues/344

packages/plant_snmp.yaml

@@ -492,6 +492,10 @@ sensor:
       {% set m = value | regex_findall('(?s)drivetemp-scsi-4-30\n.*?temp1:\s*\+?(-?\d+\.?\d*)') %}
       {{ m | first | float if m }}
 
+  # Legacy snmp platform pollers can WEDGE SILENTLY (no errors logged, the
+  # poller just stops — tsys9 drive went 12h stale 2026-09-04 while sibling
+  # pollers on the same OID kept working). Registry disable/enable does NOT
+  # recreate the poller; a core restart (packages/** pipeline) is required.
   - platform: snmp
     name: "PFV tsys9 drive temperature raw"
     host: pfv-tsys9.knel.net