[Serverless] Document impact of using logsDB for security users (#6221)

* First draft

* Fixes ref

* Some light edits for grammar and clarity.

* Fixing broken ref link

* Input from Kseniia and Vitalii

* Fix links

* Kseniia's initial feedback

* Update docs/serverless/rules/detections-logsdb-impact.asciidoc

* Removed extra word

* Update docs/serverless/rules/detections-logsdb-impact.asciidoc

Co-authored-by: Janeen Mikell Roberts <[email protected]>

* Update docs/serverless/rules/detections-logsdb-impact.asciidoc

Co-authored-by: Janeen Mikell Roberts <[email protected]>

* Update docs/serverless/rules/detections-logsdb-impact.asciidoc

Co-authored-by: Janeen Mikell Roberts <[email protected]>

* Update docs/serverless/rules/detections-logsdb-impact.asciidoc

---------

Co-authored-by: Janeen Roberts <[email protected]>
Co-authored-by: Janeen Mikell Roberts <[email protected]>

docs/serverless/index.asciidoc

@@ -130,6 +130,7 @@ include::./dashboards/rule-monitoring-dashboard.asciidoc[leveloffset=+3]
 
 include::./rules/detection-engine-overview.asciidoc[leveloffset=+2]
 include::./rules/detections-permissions-section.asciidoc[leveloffset=+3]
+include::./rules/detections-logsdb-impact.asciidoc[leveloffset=+3]
 
 include::./rules/about-rules.asciidoc[leveloffset=+2]
 include::./rules/rules-ui-create.asciidoc[leveloffset=+3]

docs/serverless/rules/detection-engine-overview.asciidoc

@@ -141,3 +141,9 @@ Refer to <<enable-detections-ui,Enable and access detections>> for a list of all
 If you get this message, you do not have the
 <<detections-permissions,required privileges>> to view the **Detections** feature,
 and you should contact your project administrator.
+
+[discrete]
+[[detections-logsdb-index-mode]]
+== Using logsDB index mode 
+
+LogsDB is enabled by default for Elastic serverless. Refer to <<detections-logsdb-index-mode-impact>> to learn more. 

docs/serverless/rules/detections-logsdb-impact.asciidoc

@@ -0,0 +1,63 @@
+[[detections-logsdb-index-mode-impact]]
+= Using logsDB index mode with {sec-serverless}
+
+LogsDB is enabled by default for {serverless-full}. This topic explains the impact of using logsDB index mode with  {sec-serverless}.  
+
+With logsDB index mode, the original `_source` field is not stored in the index but can be reconstructed using {ref}/mapping-source-field.html#synthetic-source[synthetic `_source`].
+
+When the `_source` is reconstructed, {ref}/mapping-source-field.html#synthetic-source-modifications[modifications] are possible. Therefore, there could be a mismatch between user's expectations and how fields are formatted.
+
+Continue reading to find out how this affects specific {sec-serverless} components. 
+
+[discrete]
+[[logsdb-alerts]]
+== Alerts
+
+When alerts are generated, the `_source` event is copied into the alert to retain the original data. When the logsDB index mode is applied, the `_source` event stored in the alert is reconstructed using synthetic `_source`.
+
+If you're switching to use logsDB index mode, the `_source` field stored in the alert might look different in certain situations:
+
+* {ref}/mapping-source-field.html#synthetic-source-modifications-leaf-arrays[Arrays can be reconstructed differently or deduplicated]
+* {ref}/mapping-source-field.html#synthetic-source-modifications-field-names[Field names] 
+* `geo_point` data fields (refer to {ref}/mapping-source-field.html#synthetic-source-modifications-ranges[Representation of ranges] and {ref}/mapping-source-field.html#synthetic-source-precision-loss-for-point-types[Reduced precision of `geo_point` values] for more information)
+
+Alerts generated by the following rule types could be affected:
+
+* Custom query
+* Event correlation (non-sequence only)
+* Non-aggregate rule types (for example, {esql} rules that use non-aggregating queries)
+
+Alerts that are generated by threshold, {ml}, and event correlation sequence rules are not affected since they do not contain copies of the original source.
+
+[discrete]
+[[logsdb-rule-actions]]
+== Rule actions
+
+While we do not recommend using `_source` for actions, in cases where the action relies on the `_source`, the same limitations and changes apply.
+
+If you send alert notifications by enabling {kibana-ref}/alerting-getting-started.html#alerting-concepts-actions[actions] to the external systems that have workflows or automations based on fields formatted from the original source, they may be affected. In particular, this can happen when the fields used are arrays of objects.
+
+We recommend checking and adjusting the rule actions using `_source` before switching to logsDB index mode.
+
+[discrete]
+[[logsdb-runtime-fields]]
+== Runtime fields
+
+Runtime fields that reference `_source` may be affected. Some runtime fields might not work and need to be adjusted. For example, if an event was indexed with the value of `agent.name` in the dot-notation form, it will be returned in the nested form and might not work. 
+
+The following is an example of accessing `_source` that works with the logsDB index mode enabled:
+
+[source,console]
+----
+"source": """  emit(params._source.agent.name + "_____" + doc['agent.name'].value ); """ 
+"source": """  emit(params._source['agent']['name'] + "_____" + doc['agent.name'].value );  """
+"source": """  emit(field('agent.name').get(null) + "_____" + doc['agent.name'].value ); """
+"source": """  emit($('agent.name', null) + "_____" + doc['agent.name'].value ); """
+----
+
+The following will not work with synthetic source (logsDB index mode enabled):
+
+[source,console]
+----
+"source": """  emit(params._source['agent.name'] + "_____" + doc['agent.name'].value );  """
+----