[DOCS] Searches on the `_type` field are no longer supported (#78400)

Adds an 8.0 breaking change for PR #68564

docs/reference/migration/migrate_8_0/search.asciidoc

@@ -6,6 +6,23 @@
 //Installation and Upgrade Guide
 
 //tag::notable-breaking-changes[]
+[[_type-search-matches-no-docs]]
+.Searches on the `_type` field are no longer supported.
+[%collapsible]
+====
+*Details* +
+In 8.x, the `_type` metadata field has been removed. {es} now handles a search
+on the `_type` field as a search on a non-existent field. A search on a
+non-existent field matches no documents, regardless of the query string.
+
+In 7.x, a search for `_doc` in the `_type` field would match the same documents
+as a `match_all` query.
+
+*Impact* +
+Remove queries on the `_type` field from your search requests and search
+templates. Searches that include these queries may return no results.
+====
+
 [[msearch-empty-line-support]]
 .The multi search API now parses an empty first line as action metadata in text files.
 [%collapsible]