Tables: comments for saving fix when using ImGuiTableFlags_Sortable and no user changes were made. (#9519, #9108)

Because SortOrder save is gated, it doesn't guarantee every column gets an .ini entry. Amend 1d7b37dc9b.
We set ImGuiTableFlags_Reorderable but technically any other field would work, tho ImGuiTableFlags_Reorderable makes more sense.