Cross link and clean up API descriptions content

What does this MR do?

There are currently no detailed instructions in the REST API doc guide for how to write descriptions: https://docs.gitlab.com/ee/development/documentation/restful_api_styleguide.html#method-description

There are some instructions in the GraphQL API style guide: https://docs.gitlab.com/ee/development/api_graphql_styleguide.html#description-style-guide

We should be using a consistent set of instructions for both. This MR:

• Links from the REST API docs to the GraphQL description style guide to create a single source of truth.
• Cleans up the examples to make them more up to date and consistent with each other.
• Organises the GraphQL description style guide into subsections, instead of using a long list of rules that's hard to scan.

Preview:

• http://main-ee-84063.35.193.151.162.nip.io/ee/development/documentation/restful_api_styleguide.html#method-description
• http://main-ee-84063.35.193.151.162.nip.io/ee/development/api_graphql_styleguide.html#description-style-guide

Related issues

#355873 (closed)

Author's checklist

• Consider taking the GitLab Technical Writing Fundamentals course.
• Follow the:
  • Documentation process.
  • Documentation guidelines.
  • Style Guide.
• Ensure that the product tier badge is added to topic's h1.
• Request a review based on:
  • The documentation page's metadata.
  • The associated Technical Writer.

If you are 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.

Review 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 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.
  • 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.
• Ensure a release milestone is set.