cdc: add compatibility notes for sort-dir (#6086)

ticdc/ticdc-overview.md

@@ -129,3 +129,13 @@ For details, refer to [Troubleshoot TiCDC](/ticdc/troubleshoot-ticdc.md).
 ## TiCDC Open Protocol
 
 TiCDC Open Protocol is a row-level data change notification protocol that provides data sources for monitoring, caching, full-text indexing, analysis engines, and primary-secondary replication between different databases. TiCDC complies with TiCDC Open Protocol and replicates data changes of TiDB to third-party data medium such as MQ (Message Queue). For more information, see [TiCDC Open Protocol](/ticdc/ticdc-open-protocol.md).
+
+## Compatibility notes for `sort-dir` and `data-dir`
+
+The `sort-dir` configuration is used to specify the temporary file directory for the TiCDC sorter. Its functionalities might vary in different versions. The following table lists `sort-dir`'s compatibility changes across versions.
+
+| Version | `sort-engine` functionality | Note | Recommendation |
+| :--- | :--- | :-- | :-- |
+| v4.0.11 or an earlier v4.0 version, v5.0.0-rc | It is a changefeed configuration item and specifies temporary file directory for the `file` sorter and `unified` sorter. | In these versions, `file` sorter and `unified` sorter are **experimental features** and **NOT** recommended for the production environment. <br/><br/> If multiple changefeeds use the `unified` sorter as its `sort-engine`, the actual temporary file directory might be the `sort-dir` configuration of any changefeed, and the directory used for each TiCDC node might be different. | It is not recommended to use `unified` sorter in the production environment. |
+| v4.0.12, v4.0.13, v5.0.0, and v5.0.1 | It is a configuration item of changefeed or of `cdc server`. | By default, the `sort-dir` configuration of a changefeed does not take effect, and the `sort-dir` configuration of `cdc server` defaults to `/tmp/cdc_sort`. It is recommended to only configure `cdc server` in the production environment.<br /><br /> If you use TiUP to deploy TiCDC, it is recommended to use the latest TiUP version and set `sorter.sort-dir` in the TiCDC server configuration.<br /><br /> The `unified` sorter is enabled by default in v4.0.13, v5.0.0, and v5.0.1. If you want to upgrade your cluster to these versions, make sure that you have correctly configured `sorter.sort-dir` in the TiCDC server configuration. | You need to configure `sort-dir` using the `cdc server` command-line parameter (or TiUP). |
+| v4.0.14 and later v4.0 versions, v5.0.2 and later v5.0 versions, later TiDB versions | `sort-dir` is deprecated. It is recommended to configure `data-dir`. | You can configure `data-dir` using the latest version of TiUP. In these TiDB versions, `unified` sorter is enabled by default. Make sure that `data-dir` has been configured correctly when you upgrade your cluster. Otherwise, `/tmp/cdc_data` will be used by default as the temporary file directory. <br /><br /> If the storage capacity of the device where the directory is located is insufficient, the problem of insufficient hard disk space might occur. In this situation, the previous `sort-dir` configuration of changefeed will become invalid.| You need to configure `data-dir` using the `cdc server` command-line parameter (or TiUP). |