Moved warning down to be more visible

Why is this change being made?

When users search for GitLab Markdown, this page comes up first, when we'd prefer that the GitLab Flavored Markdown page be returned instead. This MR changes the wording of the warning slightly and moves it below the heading so it might call better attention to it.

Related to: #13031 (closed)

Before	After

Author Checklist

• Provided a concise title for the MR
• Added a description to this MR explaining the reasons for the proposed change, per say-why-not-just-what
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• Assign reviewers for this change to the correct DRI(s)
  • If the DRI for the page/s being updated isn’t immediately clear, then assign it to one of the people listed in the "Maintained by" section in on the page being edited.
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• If the changes affect team members, or warrant an announcement in another way, please consider posting an update in #whats-happening-at-gitlab linking to this MR.
  • If this is a change that directly impacts the majority of global team members, it should be a candidate for #company-fyi. Please work with internal communications and check the handbook for examples.

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