system-variable: indicate that max_allowed_packet is ready only in th…

…e session scope (#15946)

system-variables.md

@@ -656,6 +656,7 @@ This variable is an alias for [`last_insert_id`](#last_insert_id).
 - Range: `[1024, 1073741824]`
 - The value should be an integer multiple of 1024. If the value is not divisible by 1024, a warning will be prompted and the value will be rounded down. For example, when the value is set to 1025, the actual value in TiDB is 1024.
 - The maximum packet size allowed by the server and the client in one transmission of packets.
+- In the `SESSION` scope, this variable is read-only.
 - This variable is compatible with MySQL.
 
 ### password_history <span class="version-mark">New in v6.5.0</span>