[DOCS] Refer back to index API for full-document updates in _update API section (#28677)

This clarifies how full-document updates are performed in ES.

docs/reference/docs/update.asciidoc

@@ -118,8 +118,11 @@ POST test/_doc/1/_update
 
 The update API also support passing a partial document,
 which will be merged into the existing document (simple recursive merge,
-inner merging of objects, replacing core "keys/values" and arrays). For
-example:
+inner merging of objects, replacing core "keys/values" and arrays).
+To fully replace the existing document, the <<docs-index_,`index` API>> should
+be used instead.
+The following partial update adds a new field to the
+existing document:
 
 [source,js]
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