Add documentation on ttlSecondsAfterFinished

Signed-off-by: Jeremy Solbrig <[email protected]>

docs/user-guide/how_to_use_job_ttl.md

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+# How to Configure Volcano Job Time to Live
+## Background
+Similar to a standard `Job` resource, VolcanoJobs can be configured to be automatically garbage 
+collected after they finish execution (either Complete or Failed). This is configured by setting 
+`spec.ttlSecondsAfterFinished` which limits the lifetime of a Job.
+
+## Key Points
+`ttlSecondsAfterFinished` is an optional parameter that can be configured on VolcanoJobs which 
+defaults to `nil`. The value of `ttlSecondsAfterFinished` must be a positive integer and indicates 
+the number of seconds after a job finishes executing (either Complete or Failed) before it becomes 
+eligible for garbage collection.
+
+If `ttlSecondsAfterFinished` is unset or set to `nil`, the job will remain indefinitely. If set to 
+zero (`0`), the job will become eligible for garbage collection immediately upon completion. If set 
+to a positive integer, `N`, the job will become eligible for garbage collection `N` seconds after 
+the job has completed.
+
+## Other Reading
+While this uses a custom garbage collector, this operates nearly identically to 
+`ttlSecondsAfterFinished` from a standard `batch.v1.job` resource. The [official Kubernetes 
+documentation](https://kubernetes.io/docs/concepts/workloads/controllers/ttlafterfinished/) has some 
+useful tips describing how mutating webhooks can be used to take greater advantage of 
+`ttlSecondsAfterFinished`.
+
+## Example
+The manifest below creates a job that will be eligible for garbage collection 10 minutes after it 
+either completes or fails.
+
+```yaml
+apiVersion: batch.volcano.sh/v1alpha1
+kind: job
+metadata:
+  generateName: test-job-
+spec:
+  minAvailable: 1
+  schedulerName: volcano
+  queue: testing
+  ttlSecondsAfterFinished: 600
+  policies:
+    - event: PodEvicted
+      action: RestartJob
+  tasks:
+    - replicas: 1
+      name: sleeper
+      policies:
+        - event: TaskCompleted
+          action: CompleteJob
+      template:
+        spec:
+          restartPolicy: Never
+          imagePullPolicy: IfNotPresent
+          containers:
+            - name: sleeper
+              image: debian:buster
+              command:
+                - /bin/bash
+                - -c
+                - |
+                  for i in {0..5}; do
+                      echo "sleeping"
+                      sleep 1
+                  done
+```