Add note to migration docs on silent batch mode (#29365)

A previous change removed silent batch mode for the plugin
installer. This commit adds a note to the migration docs regarding this
change.

Author: jasontedor

docs/reference/migration/migrate_6_3.asciidoc

@@ -17,4 +17,15 @@ See {plugins}/repository-gcs-client.html#repository-gcs-client[Google Cloud Stor
 save disk space. As a consequence, database files had to be loaded in memory. Now the default database files
 that are stored uncompressed as `.mmdb` files which allows to memory-map them and save heap memory. Any
 custom database files must also be stored uncompressed. Consequently, the `database_file` property in any
-ingest pipelines that use the Geoip Processor must refer to the uncompressed database files as well.
+ingest pipelines that use the Geoip Processor must refer to the uncompressed database files as well.
+
+==== Using the plugin installer without a TTY
+
+The Elasticsearch plugin installer (`elasticsearch-plugin install`) would
+previously silently accept additional security permissions required by a plugin
+if standard input was closed or there was no TTY attached (e.g., `docker exec
+<container ID> elasticsearch-plugin install`). This silent accepting of
+additional security permissions has been removed. Now, a user must deliberately
+accept these permissions either by keeping standard input open and attaching a
+TTY (i.e., using interactive mode to accept the permissions), or by passing the
+`--batch` flag.