[DOCS] Always enable file and native realms unless explicitly disabled (#78405)

* [DOCS] Always enable file and native realms by default

Adds an 8.0 breaking change for PR #69096.

The copy is based on the 7.13 deprecation notice added with PR #69320.

* reword

* Update docs/reference/migration/migrate_8_0/security.asciidoc

Co-authored-by: Yang Wang <ywangd@gmail.com>

* Update docs/reference/migration/migrate_8_0/security.asciidoc

Co-authored-by: Yang Wang <ywangd@gmail.com>

Co-authored-by: Yang Wang <ywangd@gmail.com>

docs/reference/migration/migrate_8_0/security.asciidoc

@@ -6,6 +6,40 @@
 //Installation and Upgrade Guide
 
 //tag::notable-breaking-changes[]
+.The file and native realms are now enabled unless explicitly disabled.
+[%collapsible]
+====
+*Details* +
+The file and native realms are now enabled unless explicitly disabled. If
+explicitly disabled, the file and native realms remain disabled at all times.
+
+Previously, the file and native realms had the following implicit behaviors:
+
+* If the file and native realms were not configured, they were implicitly disabled
+if any other realm was configured.
+
+* If no other realm was available because realms were either not configured,
+not perrmitted by license, or explicitly disabled, the file and native realms 
+were enabled, even if explicitly disabled.
+
+*Impact* +
+To explicilty disable the file or native realm, set the respective
+`file.<realm-name>.enabled` or `native.<realm-name>.enabled` setting to `false`
+under the `xpack.security.authc.realms` namespace in `elasticsearch.yml`.
+
+The following configuration example disables the native realm and the file realm.
+
+[source,yaml]
+----
+xpack.security.authc.realms:
+  
+  native.realm1.enabled: false
+  file.realm2.enabled: false
+
+  ...
+----
+====
+
 .The realm `order` setting is now required.
 [%collapsible]
 ====