Minor improvements to the `ArcanistLintCheckstyleXMLRenderer` class.

Summary: Ref T4948. Move the `startDocument` code to a `renderPreamble` function so that, at least theoretically, the renderer can be reused. Otherwise, the only way to reuse the renderer would be to construct a new instance.

Test Plan: Ran `arc lint --output xml` and verified that the output looked reasonable.

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: epriestley, Korvin

Maniphest Tasks: T4948

Differential Revision: https://secure.phabricator.com/D9108