docs: Clarifies user login process in documentation

Signed-off-by: -LAN- <laipz8200@outlook.com>

api/extensions/ext_login.py

@@ -56,7 +56,11 @@ def load_user_from_request(request_from_flask_login):
 @user_logged_in.connect
 @user_loaded_from_request.connect
 def on_user_logged_in(_sender, user):
-    """Called when a user logged in."""
+    """Called when a user logged in.
+    
+    Note: AccountService.load_logged_in_account will populate user.current_tenant_id
+    through the load_user method, which calls account.set_tenant_id().
+    """
     if user and isinstance(user, Account) and user.current_tenant_id:
         contexts.tenant_id.set(user.current_tenant_id)