Provide explanation of dangling indices, fixes #26008 (#26999)

* Provide explanation of dangling indices, fixes #26008

Adjusted from PR review comments

* updates to suggested wording and minor typo fix.

Author: geekpete

docs/reference/modules/gateway.asciidoc

@@ -48,3 +48,12 @@ as long as the following conditions are met:
     Recover as long as this many data nodes have joined the cluster.
 
 NOTE: These settings only take effect on a full cluster restart.
+
+=== Dangling indices
+
+When a node joins the cluster, any shards stored in its local data directory 
+directory which do not already exist in the cluster will be imported into the 
+cluster. This functionality is intended as a best effort to help users who 
+lose all master nodes. If a new master node is started which is unaware of 
+the other indices in the cluster, adding the old nodes will cause the old 
+indices to be imported, instead of being deleted.