util/alloc_failure_injection: fixed doxygen comments
util/alloc_failure_injection: fixed doxygen comments

Namely:
* Fix code block in the commend of `alloc_failure_injector`.
* Fix comment markers of `alloc_failure_injector` members as well as
  `on_alloc_point()` so doxygen
  recognizes them as documentation comments.
* Add missing comment to `alloc_failure_injection::on_alloc_point()`,
 and `local_failure_injector()`.
* Mark `the_alloc_failure_injector` as internal.

dreamsxin/seastar,syuu1228/seastar,cloudius-systems/seastar,avikivity/seastar,scylladb/seastar,syuu1228/seastar,dreamsxin/seastar,dreamsxin/seastar,avikivity/seastar,cloudius-systems/seastar,cloudius-systems/seastar,scylladb/seastar,avikivity/seastar,syuu1228/seastar,scylladb/seastar