Document index setting around source metadata field. (#127000)

Author: martijnvg

docs/reference/elasticsearch/index-settings/source.md

@@ -0,0 +1,24 @@
+---
+mapped_pages:
+  - https://www.elastic.co/guide/en/elasticsearch/reference/current/source-index-settings.html
+navigation_title: Source settings
+---
+
+# Source index settings [source-index-settings]
+
+All settings around the _source metadata field.
+
+$$$source-mode$$$
+
+`index.source.mode`
+: (Static, string) The source mode for the index. Valid values are [`synthetic`](/reference/elasticsearch/mapping-reference/mapping-source-field.md#synthetic-source), [`disabled`](/reference/elasticsearch/mapping-reference/mapping-source-field.md#disable-source-field) or `stored`. Defaults to `stored`. The `stored` source mode always stores the source metadata field on disk.
+
+$$$recovery-use_synthetic_source$$$
+
+`index.recovery.use_synthetic_source`
+: (Static, boolean) If synthetic source mode is used, whether the recovery source should also be synthesized instead of stored to disk. Defaults to `true`. This setting can only be configured if synthetic source mode is enabled.
+
+$$$synthetic-source-keep$$$
+
+`index.mapping.synthetic_source_keep`
+: (Static, string) Controls how to retain accuracy of fields at the index level. Valid values are `none` or `arrays`.This is a subset of [synthetic source keep mapping attribute](/reference/elasticsearch/mapping-reference/mapping-source-field.md#synthetic-source-keep). Defaults to `arrays` if `index.mode` is `logsdb` or otherwise `none`.

docs/reference/elasticsearch/toc.yml

@@ -57,6 +57,7 @@ toc:
           children:
             - file: index-settings/preloading-data-into-file-system-cache.md
         - file: index-settings/time-series.md
+        - file: index-settings/source.md
         - file: index-settings/translog.md
   - file: index-lifecycle-actions/index.md
     children:
@@ -203,4 +204,4 @@ toc:
       - file: command-line-tools/setup-passwords.md
       - file: command-line-tools/shard-tool.md
       - file: command-line-tools/syskeygen.md
-      - file: command-line-tools/users-command.md
+      - file: command-line-tools/users-command.md