There are quite a lot of new modules in Drupal 8.x without any hook_help() text at all. Also, many of the existing modules' functionality has changed or been split up, and the hook_help() text does not reflect this.
All of the help text needs to be reviewed and/or rewritten. I've set up this as a meta-issue to go module by module and review the help text, and file issues where it needs rewriting or needs to be added.