Document breaking change around _id field data.

Author: jtibshirani

docs/reference/migration/migrate_7_6.asciidoc

@@ -35,7 +35,7 @@ specify a role descriptor with no privileges:
 [source,js]
 ----
 ...
-"role_descriptors": { 
+"role_descriptors": {
     "no-privilege": {
     }
 }
@@ -49,6 +49,14 @@ specify a role descriptor with no privileges:
 [[breaking_76_search_changes]]
 === Search changes
 
+[discrete]
+==== Aggregating and sorting on `_id` is deprecated
+It's possible to aggregate and sort on the built-in `_id` field by loading an
+expensive data structure called fielddata. This was deprecated in 7.6 and will
+be disallowed by default in 8.0. As an alternative, the `_id` field's contents
+can be duplicated into another field with docvalues enabled (note that this
+does not apply to auto-generated IDs).
+
 [discrete]
 ==== Deprecation of sparse vector fields
 The `sparse_vector` field type has been deprecated and will be removed in 8.0.

docs/reference/release-notes/7.6.asciidoc

@@ -189,11 +189,6 @@ This issue is fixed in {es} 7.7.0 and later versions (issue: {es-issue}50916[#50
 [discrete]
 === Breaking changes
 
-Mapping::
-* Add a cluster setting to disallow loading fielddata on _id field {es-pull}49166[#49166] (issues: {es-issue}26472[#26472], {es-issue}43599[#43599])
-
-
-
 [[breaking-java-7.6.0]]
 [discrete]
 === Breaking Java changes
@@ -228,6 +223,7 @@ Reindex::
 * Deprecate sorting in reindex {es-pull}49458[#49458] (issue: {es-issue}47567[#47567])
 
 Search::
+* Deprecate loading fielddata on _id field {es-pull}49166[#49166] (issues: {es-issue}26472[#26472], {es-issue}43599[#43599])
 * Update the signature of vector script functions. {es-pull}48604[#48604]
 * Deprecate the sparse_vector field type. {es-pull}48315[#48315]
 * Add a deprecation warning regarding allocation awareness in search request {es-pull}48351[#48351] (issue: {es-issue}43453[#43453])