WIP: api settings cleanup

See more details in: https://gitlab.com/gitlab-org/gitlab-ce/issues/49155

What does this MR do?

This MR will be used to collect the fixes to the various api settings files and docs (CE ONLY) that I find. It may be recreated at some point before final merge, and may need review by several people due to the wide variety of issues I've found.

File: app/helpers/application_settings_helper.rb

Order settings alphabetically

File: lib/api/settings.rb

Fix typos:
:sidekiq_throttling_queus
:performance_bar_allowed_group_id, type: String, desc: 'Depreated:

File: doc/api/settings.md
Large Cleanup of table spacing, to make .md easier to read, and to make future updates simpler.
Order settings alphabetically
add missing help_page_text setting

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Conform by the code review guidelines
  • Has been reviewed by a UX Designer
  • Has been reviewed by a Frontend maintainer
  • Has been reviewed by a Backend maintainer
  • Has been reviewed by a Database specialist
• Conform by the merge request performance guides
• Conform by the style guides
• Conform by the database guides
• If you have multiple commits, please combine them into a few logically organized commits by squashing them
• Internationalization required/considered
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?