Improve API docs for `List issues that close on merge`

What does this MR do and why?

Contributes to #433318.

Improve API docs for List project merge requests.

• Slight edits to supported request attributes
• Add tables with response attributes (1 for internal tracker, 1 for external)
• Specify return code

Documentation for the fields was sourced from:

• Existing docs
• /lib/api/entities/issue_basic.rb
• /app/graphql/types/issue_type.rb

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

How to set up and validate locally

1. Run this curl command to see the API response for a list of project MRs:

curl --header "PRIVATE-TOKEN: <your-token>" \
  --url "https://gitlab.com/api/v4/projects/45308633/merge_requests/18/closes_issues" \
  | jq  

2. Compare the response structure to what's documented by this MR