[humble] improve docu and type names of `rclc_executor_semantics_t`

[JanStaschulat]

          no backport on humble, because additional data structures were only added in Iron and Rolling. 

TODO:

• [ ] - update documentation for rclc_executor_semantics_t
• [ ] - improve type names for rclc_executor_semantics_t

Originally posted by @JanStaschulat in https://github.com/ros2/rclc/issues/383#issuecomment-1609517402