Help: Add notes for topic 'CMakePackageConfigHelpers-INSTALL_PREFIX'

Help/release/dev/CMakePackageConfigHelpers-INSTALL_PREFIX.rst

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+CMakePackageConfigHelpers-INSTALL_PREFIX
+----------------------------------------
+
+* The :module:`CMakePackageConfigHelpers` module
+  :command:`configure_package_config_file` command learned a new
+  ``INSTALL_PREFIX`` option to generate package configuration files
+  meant for a prefix other than :variable:`CMAKE_INSTALL_PREFIX`.