docs: document ZFS pool health alerting; fix pushover sound+url_title

docs/THRESHOLD_ALERTING.md

@@ -256,6 +256,55 @@ disk_monitor:
         operator: "<"
 ```
 
+### ZFS Monitor
+
+ZFS pool health is checked automatically for every pool. A pool in any state
+other than `ONLINE` (e.g. `DEGRADED`, `SUSPENDED`, `FAULTED`, `UNAVAIL`) raises
+a **CRITICAL** alert by default — no configuration required.
+
+The default threshold is equivalent to:
+
+```yaml
+zfs_monitor:
+  pools:
+    '*':
+      health_ok:
+        critical: 1
+        operator: "<"
+        hysteresis: 0.0
+        display: "ZFS pool {pool_name} is {health}"
+```
+
+`'*'` matches every pool on the host. The notification message includes the pool
+name and its current health string, e.g. `ZFS pool tank is DEGRADED`.
+
+**Override for specific pools** — named pool entries take priority over `'*'`:
+
+```yaml
+zfs_monitor:
+  pools:
+    # Suppress health alerts for a scratch pool (not mission-critical)
+    scratch:
+      health_ok:
+        enabled: false
+
+    # Capacity threshold for a specific pool
+    tank:
+      capacity:
+        warning: 75.0
+        critical: 90.0
+        operator: ">"
+        hysteresis: 0.05
+```
+
+**Alert state paths** follow the pattern `zfs_monitor.<pool_name>.health_ok`,
+so acknowledgements and silences target individual pools:
+
+```
+zfs_monitor.tank.health_ok
+zfs_monitor.backup.health_ok
+```
+
 ### Network Monitor
 
 ```yaml