[DOCS] Added caveat about the number of file realms (#58369)

x-pack/docs/en/security/authentication/file-realm.asciidoc

@@ -21,7 +21,8 @@ credentials of all other users.
 
 IMPORTANT: When you configure realms in `elasticsearch.yml`, only the realms you
 specify are used for authentication. To use the `file` realm you must explicitly
-include it in the realm chain.
+include it in the realm chain. While it is possible to define multiple instances
+of some other realms, you can define only _one_ file realm per node.
 
 To define users, the {security-features} provide the
 <<users-command,users>> command-line tool. This tool enables you to add