Improve the documentation of Index Propagation in Confluence DC cluster, how does index will be propagated

Issue Summary

The system administrator should know its mechanism for troubleshooting how index progated in cluster.
There is no technical description about Index Propagation, especially the index propagation will be executed via file system in Shared Home Directory.

This is reproducible on Data Center: yes

Suggest

Add the detail of how index will be propagated in cluster like "via Shared Home Directory", because the system administrator should know its mechanism for troubleshooting how index progated in cluster.

ex: The index files will only be propagated to nodes that have joined the cluster via Shared Home Directory. If Confluence is not running on a node, we won't be able to propagate the index to that node.

Actual Results

Propagate the search index to your cluster
> The index files will only be propagated to nodes that have joined the cluster. If Confluence is not running on a node, we won't be able to propagate the index to that node.

Workaround

Ask Atlassian support