Update TiDB configuration max-index-length (pingcap#21751) (pingcap#21761)

Author: ti-chi-bot

tidb-configuration-file.md

@@ -138,7 +138,13 @@ The TiDB configuration file supports more options than command-line parameters.
 - Sets the maximum allowable length of the newly created index.
 - Default value: `3072`
 - Unit: byte
-- Currently, the valid value range is `[3072, 3072*4]`. MySQL and TiDB (version < v3.0.11) do not have this configuration item, but both limit the length of the newly created index. This limit in MySQL is `3072`. In TiDB (version =< 3.0.7), this limit is `3072*4`. In TiDB (3.0.7 < version < 3.0.11), this limit is `3072`. This configuration is added to be compatible with MySQL and earlier versions of TiDB.
+- Range: `[3072, 3072*4]`
+- Compatibility:
+    - MySQL: the maximum index length is fixed at 3072 bytes.
+    - Earlier versions of TiDB:
+        - v3.0.7 and earlier: the maximum index length is fixed at 3072 × 4 bytes.
+        - v3.0.8 ~ v3.0.10: the maximum index length is fixed at 3072 bytes.
+    - v3.0.11 and later versions: TiDB introduces the `max-index-length` configuration item to ensure compatibility with different TiDB versions and with MySQL.
 
 ### `table-column-count-limit` <span class="version-mark">New in v5.0</span>