Help: Document the package registry in cmake-packages.7

Port documentation from the CMake Wiki page at:

 http://www.cmake.org/Wiki/CMake/Tutorials/Package_Registry

as of 2014-02-17 into our main documentation.
Help: Format find_package() command documentation

Add inline markup and explicit markup block syntax as needed.
Add cross-references to other documentation as appropriate.
Merge topic 'doc-package-registry' into next

bf012e0c Help: Format find_package() command documentation
bd6887e4 Help: Document the package registry in cmake-packages.7