For 2.1.8, deprecate (via doxygen comments) public API access to:
* `struct pe_action_wrapper_s`
* `struct pe_action_s` (except as the opaque type alias `pcmk_action_t`)
* `pcmk_on_fail_text()`
* `pcmk_action_text()`
* `pcmk_parse_action()`
* `enum pe_action_flags`
* `enum action_fail_response`
* `enum action_tasks`
For 3.0.0, we can make those internal, and rename `struct pe_action_s` to `struct pcmk__action` for type aliasing purposes.