sync-diff user need process privilege when both upstream and downstream are TiDB

[zhenemola]

Change Request

Please answer the following questions before submitting your issue. Thanks!

1. Describe what you find is inappropriate or missing in the existing docs.
   When both upstream and downstream are using TiDB, to update the safepoint of the upstream and downstream, PD address needs to be obtained. PD address is obtained by querying the information_schema.cluster_info table and user needs process privileges. Otherwise, GC may not be blocked. When the data is large, it is prone to fail the comparison due to the situation where the GC lifetime is shorter than the transaction duration.

2. Describe your suggestion or addition.
   add "PROCESS" to the list of required privileges when both upstream and downstream are TiDB .

3. Provide some reference materials (such as documents and websites) if you could.
   https://github.com/pingcap/tidb-tools/blob/v7.5.6/sync_diff_inspector/utils/pd.go#L131

[ti-chi-bot]

Welcome @zhenemola! It looks like this is your first issue to pingcap/docs 🎉