Build the reviewer information out into two subsections

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What does this MR do?

As part of finishing up #464648 (closed), I did some cleanup yesterday in !155927 (merged) to get https://docs.gitlab.com/ee/user/project/merge_requests/reviews/ ready for some more edits.

These are the edits!

The goal: build in explanations of the various review-status icons into both the reviews and approvals pages. This work affects both features. DRY is a thing, yes, but I think this info really should be in both places. I opted to not do step-focused instructions because this isn't really an action, it's part of understanding the status of a review or approval. (Tell me if you disagree on either point, please!)

Related issues

• Related to Approvals page: explain self-approval, cut word... (!155927 - merged) where I did cleanup on the reviews page yesterday.
• Closes Document icon usage in Code Review (#464648 - closed)

Author's checklist

• Optional. Consider taking the GitLab Technical Writing Fundamentals course.
• Follow the:
  • Documentation process.
  • Documentation guidelines.
  • Style Guide.
• If you're adding a new page, add the product availability details under the H1 topic title.
• If you are a GitLab team member, request a review based on:
  • The documentation page's metadata.
  • The associated Technical Writer.

If you are a GitLab team member and only adding documentation, do not add any of the following labels:

• ~"frontend"
• ~"backend"
• ~"type::bug"
• ~"database"

These labels cause the MR to be added to code verification QA issues.

Reviewer's checklist

Documentation-related MRs should be reviewed by a Technical Writer for a non-blocking review, based on Documentation Guidelines and the Style Guide.

If you aren't sure which tech writer to ask, use roulette or ask in the #docs Slack channel.

• If the content requires it, ensure the information is reviewed by a subject matter expert.
• Technical writer review items:
  • Ensure docs metadata is present and up-to-date.
  • Ensure the appropriate labels are added to this MR.
  • Ensure a release milestone is set.
  • If relevant to this MR, ensure content topic type principles are in use, including:
    • The headings should be something you'd do a Google search for. Instead of Default behavior, say something like Default behavior when you close an issue.
    • The headings (other than the page title) should be active. Instead of Configuring GDK, say something like Configure GDK.
    • Any task steps should be written as a numbered list.
    • If the content still needs to be edited for topic types, you can create a follow-up issue with the docs-technical-debt label.
• Review by assigned maintainer, who can always request/require the reviews above. Maintainer's review can occur before or after a technical writer review.