Merging a pull request (Not found in the docs)

I see that merging a pull request is possible but I couldn't find it in the docs in this repo. Is the document not updated or the feature itself is not available?

https://developer.github.com/v3/pulls/#merge-a-pull-request-merge-button

Compare the repository's documentation and available API client features with the linked GitHub API “Merge a pull request” endpoint. Determine whether merging is already supported or only undocumented, then update the relevant documentation or clarify the missing feature; no specific file or test is named in the issue.

Written by the indexing model from the issue text.