Make merger_requests documentation more consistent

What does this MR do and why?

Updates existing merge request REST API docs to match the changes we made in OpenAPI docs as discussed in !104339 (comment 1180001634) and !104339 (comment 1179054706).

It makes the following changes:

• Return -> Returns
• MR -> merge request

Related to #376941

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.

Related to #376941