Update GET MR API response for pipelines

What does this MR do?

As per the discussion in the related issue, the example response in for GET'ing a single MR from the API is out of date, and does not show all the possible response parameters. The key point is that head_pipeline is not shown, and it's not clear if head_pipeline or pipeline is the best to use. From the discussion (#233122 (comment 947912304)) it looks like head_pipeline is recommended, and pipeline could actually be deprecated (I'll leave that for a separate issue).

Another parameter is already deprecated in that example, and has this note: // Deprecated and will be removed in API v5, use merge_user instead

I've copied the style and used // Old parameter, use head_pipeline instead. as an initial MVC, next to the pipeline example, to point people towards head_pipeline.

As a followup, we may want to update all the sample responses... 🤔

Related issues

Resolves #233122 (closed)

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