add warning for system variable tidb_skip_utf8_check (#9385) (#9406)

best-practices/haproxy-best-practices.md

@@ -108,7 +108,7 @@ You can easily use HAProxy to configure and set up a load-balanced database envi
     {{< copyable "shell-regular" >}}
 
     ```bash
-    echo 'export PATH=/app/haproxy/bin:$PATH' >> /etc/profile
+    echo 'export PATH=/app/haproxy:$PATH' >> /etc/profile
     ```
 
 5. Check whether the installation is successful:

character-set-and-collation.md

@@ -375,6 +375,10 @@ If the specified character set is `utf8` or `utf8mb4`, TiDB only supports the va
 
 To disable this error reporting, use `set @@tidb_skip_utf8_check=1;` to skip the character check.
 
+> **Note:**
+>
+> If the character check is skipped, TiDB might fail to detect illegal UTF-8 characters written by the application, cause decoding errors when `ANALYZE` is executed, and introduce other unknown encoding issues. If your application cannot guarantee the validity of the written string, it is not recommended to skip the character check.
+
 ## Collation support framework
 
 The syntax support and semantic support for the collation are influenced by the [`new_collations_enabled_on_first_bootstrap`](/tidb-configuration-file.md#new_collations_enabled_on_first_bootstrap) configuration item. The syntax support and semantic support are different. The former indicates that TiDB can parse and set collations. The latter indicates that TiDB can correctly use collations when comparing strings.

system-variables.md

@@ -1999,6 +1999,10 @@ Query OK, 0 rows affected, 1 warning (0.00 sec)
 - This variable is used to set whether to skip UTF-8 validation.
 - Validating UTF-8 characters affects the performance. When you are sure that the input characters are valid UTF-8 characters, you can set the variable value to `ON`.
 
+> **Note:**
+>
+> If the character check is skipped, TiDB might fail to detect illegal UTF-8 characters written by the application, cause decoding errors when `ANALYZE` is executed, and introduce other unknown encoding issues. If your application cannot guarantee the validity of the written string, it is not recommended to skip the character check.
+
 ### tidb_slow_log_threshold
 
 - Scope: GLOBAL