Re-organize the content added to the cmake-packages(7) manual by

* commit v3.0.0-rc1~184^2 (Help: Document export(EXPORT) in the
  cmake-packages manual, 2013-12-23),

* commit v3.0.0-rc1~154^2~1 (Help: Add notes about relocatability
  of config-file packages, 2014-01-07), and

* commit v3.2.0-rc1~345^2 (Help: Warn that paths should not be used
  in INTERFACE_ build properties, 2014-11-22).

These commits broke the natural flow of the original manual and made
wording after the new content make less sense.  Move the content into
new subsections to restore the flow of the original manual and to
make explicitly the purpose of the new content.

Shorten the relocatable usage requirement "warnings".  Refer to the
new cmake-packages(7) manual subsection to reduce duplication.  Also
clarify the distinction between paths to library dependencies and
paths to their header files.