Document /health thread dump timeout system property (#8537)

* Document /health thread dump timeout system property

* use 2.536 version

* update the supported duration formats

---------

Co-authored-by: Kevin Martens <[email protected]>

Author: gbhat618

content/doc/book/managing/system-properties.adoc

@@ -1601,6 +1601,17 @@ properties:
     To make Jenkins load time 8x faster (assuming sufficient IO), increase it to 8x.
     For example, 24 CPU Jenkins controller host use this: `-Djenkins.InitReactorRunner.concurrency=192`
 
+- name: jenkins.health.HealthCheckAction.thresholdTimeout
+  tags:
+  - tuning
+  def: |
+    `PT10S` (10 seconds)
+  since: 2.536
+  description: |
+    Log a thread dump when a `/health` check exceeds this timeout to help diagnose stuck requests.
+    Useful for debugging Kubernetes liveness probe failures caused by stuck health checks.
+    The value can be specified in ISO-8601 duration format (e.g., `PT20S`) or simple format with 1-character suffix (e.g., `20s`).
+
 - name: jenkins.install.runSetupWizard
   tags:
   - feature