[DOCS] Clarify migrate guide and search request validation

Relates to  #26811

Author: javanna

core/src/test/java/org/elasticsearch/search/aggregations/bucket/adjacency/AdjacencyMatrixAggregationBuilderTests.java

@@ -43,7 +43,6 @@
 
 public class AdjacencyMatrixAggregationBuilderTests extends ESTestCase {
 
-
     public void testFilterSizeLimitation() throws Exception {
         // filter size grater than max size should thrown a exception
         QueryShardContext queryShardContext = mock(QueryShardContext.class);

docs/reference/migration/migrate_7_0/search.asciidoc

@@ -22,7 +22,14 @@ PUT /_cluster/settings
 --------------------------------------------------
 // CONSOLE
 
-=== `_search/scroll` returns `400` for invalid requests
+==== Search API returns `400` for invalid requests
 
-The `/_search/scroll` endpoint returns `400 - Bad request` when the request invalid, while it would previously 
-return `500 - Internal Server Error` in such case.
+The Search API returns `400 - Bad request` while it would previously return
+`500 - Internal Server Error` in the following cases of invalid request:
+
+*   the result window is too large
+*   sort is used in  combination with rescore
+*   the rescore window is too large
+*   the number of slices is too large
+*   keep alive for scroll is too large
+*   number of filters in the adjacency matrix aggregation is too large