Update docs/design/2021-08-18-charsets.md

docs/design/2021-08-18-charsets.md

@@ -132,7 +132,7 @@ Other behaviors that need to be dealt with:
   - Due to the internal conversion of non-utf-8-related encoding to utf8 for processing, it is not fully compatible with MySQL in some cases in terms of illegal character processing. TiDB controls its behavior through sql_mode.
 
 - Collation
-  - Support gbk_bin/gbk_chinese_ci when opening configuration item `new_collations_enabled_on_first_bootstrap`. When this configuration is not enabled, only binary sorting after non-utf-8-related is converted to utf8 is supported.
+  - Fully support `gbk_bin` and `gbk_chinese_ci` only when the config `new_collations_enabled_on_first_bootstrap` is enabled. Otherwise, it only supports gbk_utf8mb4_bin.
 
 #### Compatibility with other components