Merge pull request #3848 from vespa-engine/kkraune-patch-1

add clarification

Author: kkraune

en/operations/batch-delete.html

@@ -36,7 +36,9 @@
   <li>
     Use <a href="../reference/document-v1-api-reference.html#delete">/document/v1</a> to delete documents
     identified by a <a href="../reference/document-select-language.html">document selection</a> -
-    example dropping all documents from the <em>my_doctype</em> schema:
+    example dropping all documents from the <em>my_doctype</em> schema.
+    The <em>cluster</em> value is the ID of the content cluster in <em>services.xml</em>,
+    e.g., <code>&lt;content id="my_cluster" version="1.0"&gt;</code>:
 <pre>
 $ curl -X DELETE \
   "$ENDPOINT/document/v1/my_namespace/my_doctype/docid?selection=true&cluster=my_cluster"