pd_cluster_status doesn't have the instance filter

[choudharypankaj]

File: /release-8.1/alert-rules.md

PD_cluster_down_store_nums
Alert rule:

(sum(pd_cluster_status{type="store_down_count"}) by (instance) > 0) and (sum(etcd_server_is_leader) by (instance) > 0)

The pd_cluster_status only have the following filters:

pd_cluster_status{instance="", job="pd", type=""}

Modify the document to update the metrics

[Oreoxmt]

@lhy1024 Could you please take a look at this issue?