Help: CPack module: Several corrections and additions

• Correct the summary: Including CPack does not "build" installers. It just configures them.
• The module builds two config files, not just one.
• Explain the usage of the two new build targets, package and package_source, and how they depend on CMake generators.
• Add cross-reference to cpack-generators(7).

Backport: release
Topic-rename: doc-cpack