[DOCS] Clarify routing enforcement in docs (#53945)

Removes a mention of the `_doc` mapping type that's
no longer applicable now that mapping types are
removed/deprecated.

docs/reference/mapping/fields/routing-field.asciidoc

@@ -92,7 +92,7 @@ PUT my_index2/_doc/1 <2>
 ------------------------------
 // TEST[catch:bad_request]
 
-<1> Routing is required for `_doc` documents.
+<1> Routing is required for all documents.
 <2> This index request throws a `routing_missing_exception`.
 
 ==== Unique IDs with custom routing
@@ -128,4 +128,4 @@ less than `index.number_of_shards`.
 Once enabled, the partitioned index will have the following limitations:
 
 *   Mappings with <<parent-join,`join` field>> relationships cannot be created within it.
-*   All mappings within the index must have the `_routing` field marked as required.
+*   All mappings within the index must have the `_routing` field marked as required.