Add docs for filesystem health checks

Documents the feature and settings introduced in elastic#52680.

Author: DaveCTurner

docs/reference/modules/discovery/discovery-settings.asciidoc

@@ -227,3 +227,22 @@ APIs are not be blocked and can run on any available node.
 ===============================
 
 --
+
+`monitor.fs.health.enabled`::
+
+    (<<cluster-update-settings,Dynamic>>, boolean) Sets whether the node runs
+    periodic <<cluster-fault-detection-filesystem-health,filesystem health
+    checks>>. Defaults to `true`.
+
+`monitor.fs.health.refresh_interval`::
+
+    (<<time-units, Time unit value>>) Sets the interval between successive
+    <<cluster-fault-detection-filesystem-health,filesystem health checks>>.
+    Defaults to `2m`.
+
+`monitor.fs.health.slow_path_logging_threshold`::
+
+    (<<time-units, Time unit value>>) If a
+    <<cluster-fault-detection-filesystem-health,filesystem health checks>>
+    takes longer than this threshold then {es} logs a warning. Defaults to
+    `5s`.

docs/reference/modules/discovery/fault-detection.asciidoc

@@ -18,3 +18,10 @@ Similarly, if a node detects that the elected master has disconnected, this
 situation is treated as an immediate failure. The node bypasses the timeout and
 retry settings and restarts its discovery phase to try and find or elect a new
 master.
+
+[[cluster-fault-detection-filesystem-health]]
+Additionally, each node periodically verifies that its data path is healthy by
+writing a small file to disk and then deleting it again. If a node discovers
+its data path is unhealthy then it is removed from the cluster until the data
+path recovers. You can control this behavior with the
+<<modules-discovery-settings,`monitor.fs.health` settings>>.