docs/design: update collation compatibility issues in charsets doc

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

@@ -98,8 +98,10 @@ After receiving the non-utf-8 character set request, this solution will convert
 ### Collation
 
 Add gbk_chinese_ci and gbk_bin collations. In addition, considering the performance, we can add the collation of utf8mb4 (gbk_utf8mb4_bin).
+- To support gbk_chinese_ci and gbk_bin collations, it needs to turn on the `new_collations_enabled_on_first_bootstrap` switch.
+  - If `new_collations_enabled_on_first_bootstrap` is off, it only supports gbk_utf8mb4_bin which does not need to be converted to gbk charset before processing.
 - Implement the Collator and WildcardPattern interface functions for each collation.
-  - gbk_chinese_ci and gbk_bin need to convert utf-8 to gbk encoding and then generate a sort key. gbk_utf8mb4_bin does not need to be converted to gbk code for processing.
+  - gbk_chinese_ci and gbk_bin need to convert utf-8 to gbk encoding and then generate a sort key.
   - Implement the corresponding functions in the Coprocessor.
 
 ### DDL
@@ -119,43 +121,18 @@ Other behaviors that need to be dealt with:
 #### Compatibility between TiDB versions
 
 - Upgrade compatibility:
-  - Upgrades from versions below 4.0 do not support gbk or any character sets other than the original five (binary, ascii, latin1, utf8, utf8mb4).
-  - Upgrade from version 4.0 or higher
-    - There may be compatibility issues when performing non-utf-8-related operations during the rolling upgrade.
-    - The new version of the cluster is expected to have no compatibility issues when reading old data.
+  - There may be compatibility issues when performing  operations during the rolling upgrade.
+  - The new version of the cluster is expected to have no compatibility issues when reading old data.
 - Downgrade compatibility: 
   - Downgrade is not compatible. The index key uses the table of gbk_bin/gbk_chinese_ci. The lower version of TiDB will have problems when decoding, and it needs to be transcoded before downgrading.
 
 #### Compatibility with MySQL
 
-Illegal character related issue:
+- Illegal character related issue:
+  - 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.
 
-```sql
-create table t3(a char(10) charset gbk);
-insert into t3 values ('a');
-
-// 0xcee5 is a valid gbk hex literal but invalid utf8mb4 hex literal.
-select hex(concat(a, 0xcee5)) from t3; 
--- mysql 61cee5 
-
-// 0xe4b880 is an invalid gbk hex literal but valid utf8mb4 hex literal.
-select hex(concat(a, 0xe4b880)) from t3;
--- mysql 61e4b880 (test on mysql 5.7 and 8.0.22)
--- mysql returns "Cannot convert string '\x80' from binary to gbk" (test on mysql 8.0.25 and 8.0.26). TiDB will be compatible with this behavior.
-
-// 0x80 is a hex literal that invalid for neither gbk nor utf8mb4.
-select hex(concat(a, 0x80)) from t3;
--- mysql 6180 (test on mysql 5.7 and 8.0.22)
--- mysql returns "Cannot convert string '\x80' from binary to gbk" (test on mysql 8.0.25 and 8.0.26). TiDB will be compatible with this behavior.
-
-set @@sql_mode = '';
-insert into t3 values (0x80);
--- mysql gets a warning and insert null values (warning: "Incorrect string value: '\x80' for column 'a' at row 1")
-
-set @@sql_mode = 'STRICT_TRANS_TABLES';
-insert into t3 values (0x80);
--- mysql returns "Incorrect string value: '\x80' for column 'a' at row 1"
-```
+- 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.
 
 #### Compatibility with other components