[8.18] [8.x] Don't modify index mappings for Alerts indices (backport #6882)

docs/detections/about-rules.asciidoc

@@ -59,6 +59,8 @@ When you create a rule, you must either specify the {es} index pattens for which
 
 NOTE: To access data views, ensure you have the {kibana-ref}/data-views.html#data-views-read-only-access[required permissions].
 
+IMPORTANT: System indices, such as the alert indices, contain important configuration and internal data; do not change their mappings. Changes can lead to rule execution and alert indexing failures. Use <<runtime-fields, runtime fields>> instead, which allow you to add fields to existing alert and event documents.
+
 [float]
 [[about-notifications]]
 === Notifications

docs/detections/query-alert-indices.asciidoc

@@ -2,6 +2,8 @@
 == Query alert indices
 This page explains how you should query alert indices, for example, when building rule queries, custom dashboards, or visualizations. For more information about alert event field definitions, review the <<alert-schema, Alert schema>>.
 
+IMPORTANT: System indices, such as the alert indices, contain important configuration and internal data; do not change their mappings. Changes can lead to rule execution and alert indexing failures. Use <<runtime-fields, runtime fields>> instead, which allow you to add fields to existing alert and event documents.
+
 [discrete]
 === Alert index aliases
 We recommend querying the following index aliases:

docs/reference/alert-schema.asciidoc

@@ -9,7 +9,12 @@
 
 {elastic-sec} stores alerts that have been generated by detection rules in hidden {es} indices. In 8.x versions, the index pattern is `.alerts-security.alerts-<space-id->`. In 7.x versions, the index pattern was `.siem-signals-<space-id>`, and some field names were different. The following table includes the current names and cross-references the legacy field names.
 
-NOTE: Users are advised NOT to use the `_source` field in alert documents, but rather to use the `fields` option in the search API to programmatically obtain the list of fields used in these documents. Learn more about {ref}/search-fields.html[retrieving selected fields from a search].
+[IMPORTANT]
+====
+
+* System indices, such as the alert indices, contain important configuration and internal data; do not change their mappings. Changes can lead to rule execution and alert indexing failures. Use <<runtime-fields, runtime fields>> instead, which allow you to add fields to existing alert and event documents.
+* We recommend to NOT use the `_source` field in alert documents, but rather the `fields` option in the search API to programmatically obtain the list of fields used in these documents. Learn more about {ref}/search-fields.html[retrieving selected fields from a search].
+====
 
 NOTE: The non-ECS fields listed below are beta and subject to change.