Document the new variable `EXECUTOR_JOB_SECTION_ATTEMPTS` introduced in
https://gitlab.com/gitlab-org/gitlab-runner/-/merge_requests/1995 for
the Docker executor for a single error but we might want to add more
errors/executors for this kind of logic.

More detail about the variable can be found in
https://gitlab.com/gitlab-org/gitlab-runner/-/merge_requests/1995#note_320619026

reference https://gitlab.com/gitlab-org/gitlab-runner/-/merge_requests/1995#note_320619026
reference https://gitlab.com/gitlab-org/gitlab-runner/-/issues/4450