Remove deprecated sort options: nested_path and nested_filter

This commit removes the nested_path and nested_filter options deprecated in 6x.
This change also checks that the sort field has a [nested] option if it is under a nested
object and throws an exception if it's not the case.

Closes elastic#27098

Author: jimczi

docs/reference/migration/migrate_8_0/search.asciidoc

@@ -12,11 +12,16 @@ The `/{index}/{type}/_termvectors`, `/{index}/{type}/{id}/_termvectors` and `/{i
 [float]
 ==== Removal of queries
 
-The `common` query was deprecated in 7.x and  has been removed in 8.0.
-The same functionality can be achieved by the `match` query if the total number of hits is not tracked.
+* The `common` query was deprecated in 7.x and  has been removed in 8.0.
+  The same functionality can be achieved by the `match` query if the total number of hits is not tracked.
 
 [float]
 ===== Removal of query parameters
 
-The `cutoff_frequency` parameter was deprecated in 7.x and has been removed in 8.0 from `match` and `multi_match` queries.
-The same functionality can be achieved without any configuration provided that the total number of hits is not tracked.
+* The `cutoff_frequency` parameter was deprecated in 7.x and has been removed in 8.0 from `match` and `multi_match` queries.
+  The same functionality can be achieved without any configuration provided that the total number of hits is not tracked.
+
+[float]
+===== Removal of sort parameters
+
+* The `nested_filter` and `nested_path` options, deprecated in 6.x, has been removed in favor of the `nested` context.

docs/reference/search/request/sort.asciidoc

@@ -252,20 +252,16 @@ field support has a `nested` sort option with the following properties:
     A filter that the inner objects inside the nested path
     should match with in order for its field values to be taken into account
     by sorting. Common case is to repeat the query / filter inside the
-    nested filter or query. By default no `nested_filter` is active.
+    nested filter or query. By default no `filter` is active.
 `max_children`::
     The maximum number of children to consider per root document
     when picking the sort value. Defaults to unlimited.
 `nested`::
     Same as top-level `nested` but applies to another nested path within the
     current nested object.
 
-[WARNING]
-.Nested sort options before Elasticsearch 6.1
-============================================
-
-The `nested_path` and `nested_filter` options have been deprecated in
-favor of the options documented above.
+NOTE: Elasticsearch will throw an error if a nested field is defined in a sort without
+a `nested` context.
 
 ============================================
 
@@ -300,7 +296,7 @@ POST /_search
 // CONSOLE
 
 In the below example `parent` and `child` fields are of type `nested`.
-The `nested_path` needs to be specified at each level; otherwise, Elasticsearch doesn't know on what nested level sort values need to be captured.
+The `nested.path` needs to be specified at each level; otherwise, Elasticsearch doesn't know on what nested level sort values need to be captured.
 
 [source,js]
 --------------------------------------------------
@@ -374,7 +370,7 @@ GET /_search
 // CONSOLE
 
 NOTE: If a nested inner object doesn't match with
-the `nested_filter` then a missing value is used.
+the `nested.filter` then a missing value is used.
 
 ==== Ignoring Unmapped Fields
 

server/src/main/java/org/elasticsearch/index/search/NestedHelper.java

@@ -192,7 +192,7 @@ boolean mightMatchNonNestedDocs(String field, String nestedPath) {
         return true; // the field is not a sub field of the nested path
     }
 
-    private static String parentObject(String field) {
+    public static String parentObject(String field) {
         int lastDot = field.lastIndexOf('.');
         if (lastDot == -1) {
             return null;