[DOCS] Javadoc: delete index request does not support empty String

In 0.90.x I was able to delete all my indices from the java api by calling

    client.admin().indices().prepareDelete(new String[] {}).execute().actionGet();

However this fails in 1.0.0 with

org.elasticsearch.action.ActionRequestValidationException: Validation Failed: 1: index / indices is missing;
	at org.elasticsearch.action.ValidateActions.addValidationError(ValidateActions.java:29)
	at org.elasticsearch.action.admin.indices.delete.DeleteIndexRequest.validate(DeleteIndexRequest.java:72)
	*snip long stacktrace*

which points me to

    public ActionRequestValidationException validate() {
        ActionRequestValidationException validationException = null;
        if (indices == null || indices.length == 0) {
            validationException = addValidationError("index / indices is missing", validationException);
        }
        return validationException;
    }

So that's what now throws the error, however the documentation still says:

    /**
     * Deletes an index based on the index name.
     *
     * @param indices The indices to delete. Empty array to delete all indices.
     */
    DeleteIndexRequestBuilder prepareDelete(String... indices);

Closes #5164.
Closes #5167.
Closes #5168.

src/main/java/org/elasticsearch/action/admin/indices/delete/DeleteIndexRequest.java

@@ -47,11 +47,18 @@ public class DeleteIndexRequest extends MasterNodeOperationRequest<DeleteIndexRe
 
     /**
      * Constructs a new delete index request for the specified index.
+     *
+     * @param index The index to delete. Use "_all" to delete all indices.
      */
     public DeleteIndexRequest(String index) {
         this.indices = new String[]{index};
     }
 
+    /**
+     * Constructs a new delete index request for the specified indices.
+     *
+     * @param indices The indices to delete. Use "_all" to delete all indices.
+     */
     public DeleteIndexRequest(String... indices) {
         this.indices = indices;
     }

src/main/java/org/elasticsearch/client/IndicesAdminClient.java

@@ -276,7 +276,7 @@ public interface IndicesAdminClient {
     /**
      * Deletes an index based on the index name.
      *
-     * @param indices The indices to delete. Empty array to delete all indices.
+     * @param indices The indices to delete. Use "_all" to delete all indices.
      */
     DeleteIndexRequestBuilder prepareDelete(String... indices);