Follow-up from "Fix missing build_script step for custom executor"

The following discussion from !2227 (merged) should be addressed:

• @steveazz started a discussion: (+1 comment)

  suggestion: What do you think if we split this notice into 2 since it's covering two subjects?

  diff --git a/docs/executors/custom.md b/docs/executors/custom.md
  index d9ff6d32b..763f9f966 100644
  --- a/docs/executors/custom.md
  +++ b/docs/executors/custom.md
  @@ -231,13 +231,15 @@ order:
   1. `get_sources`
   1. `restore_cache`
   1. `download_artifacts`
  +1. `step_*`
   1. `build_script`
  +1. `step_*`
   1. `after_script`
   1. `archive_cache`
   1. `upload_artifacts_on_success` OR `upload_artifacts_on_failure`
  
   NOTE: **Note:**
  -Since GitLab Runner 13.1 dynamic build stages are also supported. They are prefixed with `step_` and are not of a predefined set. Starting 14.0 `build_script` will be replaced with `step_script` - [issue](https://gitlab.com/gitlab-org/gitlab-runner/-/issues/26426).
  +Starting GitLab Runner 14.0 `build_script` will be replaced with `step_script`, for more information check [#26426](https://gitlab.com/gitlab-org/gitlab-runner/-/issues/26426).
  
   For each stage mentioned above, the `run_exec` executable will be
   executed with:
  @@ -300,7 +302,9 @@ what the main goal of that script is.
   | `get_sources`    | Prepares the Git configuration, and clone/fetch the repository. We suggest you keep this as is since you get all of the benefits of Git strategies that GitLab provides. |
   | `restore_cache` | Extract the cache if any are defined. This expects the `gitlab-runner` binary is available in `$PATH`. |
   | `download_artifacts` | Download artifacts, if any are defined. This expects `gitlab-runner` binary is available in `$PATH`. |
  -| `build_script` | This is a combination of [`before_script`](https://docs.gitlab.com/ee/ci/yaml/#before_script-and-after_script) and [script](https://docs.gitlab.com/ee/ci/yaml/#script). |
  +| `step_*` | A set of scripts to execute which is generated by GitLab, it's possible that is never sent to the custom executor or there are multiple steps such as `step_release`, `step_a11y`. This can be some features from the `.gitlab-ci.yml` to work properly. |
  +| `build_script` | This is a combination of [`before_script`](https://docs.gitlab.com/ee/ci/yaml/#before_script-and-after_script) and [script](https://docs.gitlab.com/ee/ci/yaml/#script). Starting GitLab Runner 14.0 `build_script` will be replaced with `step_script`, for more information check [#26426](https://gitlab.com/gitlab-org/gitlab-runner/-/issues/26426). |
  +| `step_*` | A set of scripts to execute which is generated by GitLab, it's possible that is never sent to the custom executor or there are multiple steps such as `step_release`, `step_a11y`. This can be some features from the `.gitlab-ci.yml` to work properly. |
   | `after_script` | This is the [`after_script`](https://docs.gitlab.com/ee/ci/yaml/#before_script-and-after_script) defined from the job. This is always called even if any of the previous steps failed. |
   | `archive_cache` | Will create an archive of all the cache, if any are defined. |
   | `upload_artifacts_on_success` | Upload any artifacts that are defined. Only executed when `build_script` was successful. |