Add iid to MR API response

## What does this MR do?

Adds `iid` parameter to the MR API docs.

## Why was this MR needed?

It is missing.

## What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/19578

## Does this MR meet the acceptance criteria?

- [x] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)

See merge request !5468
Signed-off-by: Rémy Coutable <remy@rymai.me>