[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 #383 (comment)