[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.
(cherry picked from commit 369dba1)