document the copr-cache configurations

[dbsid]

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.
   There is cases that we need to turn the copr cache, but there are config undocumented

2. Describe your suggestion or addition.

need to add below three configs under the tikv-client in tidb configurations.

[tikv-client.copr-cache]
admission-max-ranges = 5000
admission-max-result-mb = 10
admission-min-process-ms = 1

3. Provide some reference materials (such as documents and websites) if you could.