Resolves: fdo#87245 crash because clip doc doesn't own DocumentPool

multiple documents sharing the same doc pool so can't unset its
secondary here.

Here the assumption is that SharePooledResources has always been called on Clip
or Undo docs (otherwise persumably need a flag which SharePooledResources
toggles on)

Change-Id: Ibcdbe5f698102dc683c290d3bd581c31e214765f