I think search functionality should be added to a ‘Documents’ pane, which appears when user tries to move section, add link etc:
When there is many documents it is easier to search for a document instead of choose it from the document list.
I think the same applies to ‘Choose a parent’ (Move to) dialog.
BTW: why ‘Move to’ uses a custom dialog and not the ‘Documents’ pane?
Update: ah, there is no way to choose ‘Root’ in Documents pane. That’s the reason? Maybe there should be a ‘Root’ button somewhere in Documents pane (near the New document button?) for this purpose and for the ease of navigation?