Add time unit for slow query log document

Change Request

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For other issues (related to TiDB, PD, etc), please move to other repositories.

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

1. Describe what you find is inappropriate or missing in the existing docs.
   https://docs.pingcap.com/tidb/stable/identify-slow-queries#identify-slow-queries
   In the slow query log, there are several fields about execution time, like:

# Cop_proc_avg: 0.07 Cop_proc_p90: 0.07 Cop_proc_max: 0.07 Cop_proc_addr: 172.16.5.87:20171
# Cop_wait_avg: 0 Cop_wait_p90: 0 Cop_wait_max: 0 Cop_wait_addr: 172.16.5.87:20171
# Cop_backoff_regionMiss_total_times: 200 Cop_backoff_regionMiss_total_time: 0.2 Cop_backoff_regionMiss_max_time: 0.2 Cop_backoff_regionMiss_max_addr: 127.0.0.1 Cop_backoff_regionMiss_avg_time: 0.2 Cop_backoff_regionMiss_p90_time: 0.2
# Cop_backoff_rpcPD_total_times: 200 Cop_backoff_rpcPD_total_time: 0.2 Cop_backoff_rpcPD_max_time: 0.2 Cop_backoff_rpcPD_max_addr: 127.0.0.1 Cop_backoff_rpcPD_avg_time: 0.2 Cop_backoff_rpcPD_p90_time: 0.2
# Cop_backoff_rpcTiKV_total_times: 200 Cop_backoff_rpcTiKV_total_time: 0.2 Cop_backoff_rpcTiKV_max_time: 0.2 Cop_backoff_rpcTiKV_max_addr: 127.0.0.1 Cop_backoff_rpcTiKV_avg_time: 0.2 Cop_backoff_rpcTiKV_p90_time: 0.2

But the descriptions don't show the unit. I can not tell the execution time is second or millisecond.

Cop_proc_avg: The average execution time of cop-tasks.
Cop_proc_p90: The P90 execution time of cop-tasks.
Cop_proc_max: The maximum execution time of cop-tasks.

2. Describe your suggestion or addition.
   Add the time unit for the execution time fields.

3. Provide some reference materials (documents, websites, etc) if you could.