update doc of require_secure_transport (pingcap#16569)

system-variables.md

@@ -870,6 +870,7 @@ mysql> SHOW GLOBAL VARIABLES LIKE 'max_prepared_stmt_count';
 
 - Setting this variable to `ON` requires you to connect to TiDB from a session that has TLS enabled. This helps prevent lock-out scenarios when TLS is not configured correctly.
 - This setting was previously a `tidb.toml` option (`security.require-secure-transport`), but changed to a system variable starting from TiDB v6.1.0.
+- Starting from v6.5.6, v7.1.2, v7.5.1, and v8.0.0, when Security Enhanced Mode (SEM) is enabled, setting this variable to `ON` is prohibited to avoid potential connectivity issues for users.
 
 ### skip_name_resolve <span class="version-mark">New in v5.2.0</span>