typo fix (it self -> itself) (#21781)

* typo fix.

* apply "stored field value"

* replaced "whereas" with "on the contrary"

docs/reference/search/request/stored-fields.asciidoc

@@ -40,7 +40,7 @@ GET /_search
 
 If the requested fields are not stored (`store` mapping set to `false`), they will be ignored.
 
-Field values fetched from the document it self are always returned as an array. Metadata fields like `_routing` and
+Stored field values fetched from the document itself are always returned as an array. On the contrary, metadata fields like `_routing` and
 `_parent` fields are never returned as an array.
 
 Also only leaf fields can be returned via the `field` option. So object fields can't be returned and such requests