Documentation request: update plugin Guide to JIRA High Availability and Clustering

Problem Definition

Current documentation regarding Schedule a job is not specific enough about how jobs are scheduled in cluster environment:
see page: https://developer.atlassian.com/jiradev/jira-platform/jira-data-center/plugin-guide-to-jira-high-availability-and-clustering

We need to document current status, see related JRASERVER-65632

Suggested Solution

Currently docs mention:

• Jobs only run once for each scheduled run time.
• Should only be scheduled by one node in the cluster.

Please add part covering:

• There is no guaranty that only one job will be running at specific moment of time across the cluster (Singleton).
  • For example: If job configured as interval job with 1 min interval, Job will scheduled at node1 and than 1 min later at node2, so two instances of the job will be running in the cluster.
  • This should be controlled by job code itself, possibly using Cluster Locks.

Workaround

None.