Rewordings of the description of Depends and Pre-Depends

Rewordings of the description of Depends and Pre-Depends

Reword the discussion of Depends, Pre-Depends, and circular dependencies.
Document that Pre-Depends doesn't allow circular dependencies and
improve the documentation of what Depends guarantees for postinst and
prerm scripts.  Clarify the implications of circular dependencies and
recommend against them.  Reorder some of the Pre-Depends discussion.