cmake: Clarify comment for zephyr_generated_headers library

Clean up the comment around the zephyr_generated_headers CMake library
to make it more accurate, now that the library is an interface library
rather than a custom target.

This is a follow-up to non-blocking feedback from
https://github.com/zephyrproject-rtos/zephyr/pull/92927

.

Signed-off-by: Kesavan Yogeswaran <hikes@google.com>