MR API docs: move important notes to more relevant places

What does this MR do and why?

Starting point for #350533.

In !77658 (comment 803408815) we've identified some problems in MR REST API docs page: https://docs.gitlab.com/ee/api/merge_requests.html.

The way in which we document API changes at the top of the page is confusing. Each note applies only to a subset of requests, but it became really unclear which notes apply to which requests.

From what I could find, this was done on purpose. In the past some of those were in multiple sections, but were later move to the top. I assume it was done to avoid duplicating text? In this MR I propose creating subsections with response description in Get single MR and List merge requests. In other requests that have similar responses I simply link to those sections.

How to set up and validate locally

All changes are in ee/api/merge_requests.html doc page.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.