CMakePackageConfigHelpers: Document PACKAGE_PREFIX_DIR for public use

This variable has long been provided by `configure_package_config_file`
in the `@PACKAGE_INIT@` part of the package configuration file it
generates.  Although not publicly documented, projects have come to
use it.  Now that we preserve the value across nested `find_package`
calls, it has a stable meaning.

Issue: #25827
1 file changed