graceful-wait-before-shutdown is missing from documentation

[dveeden]

Error Report

https://github.com/pingcap/tidb/blob/749a99c02027fb43406d01ed9b0f33f219a37db2/config/config.toml.example#L64-L66

This should have been in https://github.com/pingcap/docs/blob/master/tidb-configuration-file.md

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

1. What is the URL/path of the document related to this issue?

https://docs.pingcap.com/tidb/stable/tidb-configuration-file

2. How would you like to improve it?

Docs should be added for graceful-wait-before-shutdown