CMakePackageConfigHelpers: Clarify generate_apple_*_selection_file signatures

The helpers added by:

* commit 37bc3400cd (CMakePackageConfigHelpers: Add
                     generate_apple_platform_selection_file(), 2023-11-03)
* commit 4ac5a2f866 (CMakePackageConfigHelpers: Add
                     generate_apple_architecture_selection_file(), 2023-12-19)

were worded in terms of including package configuration files, but the
actual functionality is to include files *from* package configuration
files.  The included files do not themselves need to be usable as
package configuration files.

Rename the options and clarify the documentation accordingly.

Issue: #25262
Issue: #25516