For those asking, it's in section seven, additional clauses:

  Notwithstanding any other provision of this License, for material
  you add to a covered work, you may (if authorized by the copyright
  holders of that material) supplement the terms of this License with
  terms:

  [...]

  c) Prohibiting misrepresentation of the origin of that material,
  or requiring that modified versions of such material be marked in
  reasonable ways as different from the original version; or...