v3.0/reference, v2.1/reference, dev/reference: update stats related configuration

[alivxxx]

What is changed, added or deleted?

Configuration of statistics is updated.

What is the related PR or file link(s)?

• master: domain: load stats when stats lease is 0 tidb#10771
• 3.0: domain: load stats when stats lease is 0 (#10771) tidb#10811
• 2.1: domain: add a config to control updating stats tidb#10772

Which version does your change affect?

Checklist

• Add a new file to TOC.md
• No Tab spaces anywhere
• Leave a blank line both before and after a code block
• Keep the first level heading consistent with title in metadata
• Use four spaces for each level of indentation except that in TOC.md

[yikeke]

Important Update: We have added a markdownlint static check in the CI check to improve the quality of our documentation, so your unclosed PR might fail the CI check now.

First, you should merge upstream master to your branch, so the upstream changes might resolve the issues found by the CI check. Then, click details beside the ci/circleci: lint — Your tests failed on CircleCI message, and you can check the details for the failure and fix all the issues in your PR. It is recommended to install "markdownlint" extension in your VS Code editor, so it will check all potential issues that fail the CI check.

@lamxTyler If you have any problem with troubleshooting, please consult @yikeke for help. Thanks for your cooperation~

Ref: #1494

[yikeke]

LGTM. PTAL again. @zz-jason

[zz-jason]

LGTM