Docs: Update and sort reserved names

What does this MR do?

The reserved Names documentation is outdated. This MR fixes this accourding to
https://gitlab.com/gitlab-org/gitlab-ce/blob/master/lib/gitlab/path_regex.rb

@axil: If this is accepted I can also prepare a MR for EE. Waiting for your comments :)

Related issues

closes #45619 (closed)

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