Document GitLab.com puma request timeout

What does this MR do?

Document GitLab.com puma request timeout.

I don't see any usage of GITLAB_RAILS_RACK_TIMEOUT in https://ops.gitlab.net/search?utf8=%E2%9C%93&search=GITLAB_RAILS_RACK_TIMEOUT&group_id=33&project_id=139&scope=&search_code=true&snippets=false&repository_ref=master&nav_source=navbar, so presume GitLab.com has 60 request timeout as well

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