adding a warning upon 'creating' a new document on an existing page

Summary: don't remember what I changed except for add that intermediate dialog

Test Plan:
{F35155}

Ugly, ain't it?

you can edit instead, or admit that it was a mistake to take on Phriction's existing documents

Reviewers: epriestley, chad, btrahan

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T2686

Differential Revision: https://secure.phabricator.com/D5295