Add clarification to Merge Request API Docs

What does this MR do?

Add a little clarification that the "Accept MR" section is the same as "Merge MR", which is what I was initially looking for in the docs. It might help those less familiar with the API to see the two as one and the same, especially as the API itself ends with 'merge'.

Related issues

n/a

Author's checklist

• Consider taking the GitLab Technical Writing Fundamentals course.
• Follow the:
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Review checklist

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• Ensure a release milestone is set.