Add docs about ATL_CLUSTER_INTERFACE (#893)

* SCALE-140: Add docs about ATL_CLUSTER_INTERFACE

* Add link to upstream Confluence docs

---------

Co-authored-by: Yevhen Ivantsov <yivantsov@atlassian.com>

docs/docs/containers/CONFLUENCE.md

@@ -339,6 +339,12 @@ management technology, and is beyond the scope of this documentation.
 
    The time-to-live for cluster packets. Primarily of use in multicast clusters.
 
+* `ATL_CLUSTER_INTERFACE`
+
+   The network interface Confluence will use to communicate between nodes. Auto-detected by default.
+   [Override interface](https://confluence.atlassian.com/doc/data-center-troubleshooting-203618.html#DataCenterTroubleshooting-Changemulticastinterface) if Confluence has selected the wrong one.
+
+
 ##### AWS cluster settings
 
    The following should be populated from the AWS environment.