add a note for alter system set session var

Signed-off-by: xxchan <[email protected]>

operate/view-configure-runtime-parameters.mdx

@@ -96,6 +96,10 @@ For details about the `set_config()` function, see [System administration functi
 
 You can also use the [ALTER SYSTEM SET](/sql/commands/sql-alter-system) command to set a system-wide default value for a runtime parameter. This configuration will then be applied to every new session.
 
+<Note>
+`ALTER SYSTEM SET` only takes effect for new sessions. The value of the runtime parameter will stay unchanged in the current session.
+</Note>
+
 ```sql Syntax
 ALTER SYSTEM SET session_param_name TO session_param_value;
 ```