Follow-up from "macOS limitations and example update"

The following discussion from !1505 (merged) should be addressed:

• @palmerc started a discussion: (+8 comments)

  1 suggestion I have is renaming Starting GitLab Runner with launchd to Using codesign with GitLab Runner Service to make it more clear since we install that launchd plist automatically when you gitlab-runner install

Should also do an extra review to double check minor issues in the added text.