Update changelog

Author: pawankartik-elastic

docs/changelog/136703.yaml

@@ -1,12 +1,18 @@
 pr: 136703
 summary: Disable cross-cluster functionality for `_fleet/_fleet_msearch`
-area: CCS
+area: Search
 type: breaking
 issues: []
 breaking:
   title: Disable cross-cluster functionality for `_fleet/_fleet_msearch`
-  area: CCS
-  details: Please describe the details of this change for the release notes. You can
-    use asciidoc.
-  impact: Please describe the impact of this change to users
+  area: Search
+  details: |-
+    This endpoint is largely used for local searches only and is not compatible with true cross-cluster searches where
+    arbitrary number of indices and remotes can be specified. Although it is meant to accept an index parameter that
+    denotes a single searchable target, such a limitation can be bypassed through various means.
+
+    Keeping in view this endpoint's stated intent and future scope, cross-cluster functionality is being explicitly disabled.
+  impact: |-
+    This endpoint will no longer accept remote indices. Should one be provided, a top-level error is returned with
+    an appropriate explanation.
   notable: false