doc: application: clarify KCONFIG_ROOT section

#. If your application has its own kernel configuration options, add a

   line setting the location of the Kconfig file that defines them.

   **This is a somewhat advanced use case; you probably don't need this.**

   An (unlikely) advanced use case would be if your application has its own

   unique configuration **options** that are set differently depending on the

   build configuration.

   This applies if your application has its own unique configuration

   **options**, which you want to set differently depending on the build

   configuration.  If you just want to set application specific **values** for

   existing Zephyr configuration options, refer to the :makevar:`CONF_FILE`

   description above.

   If you just want to set application specific **values** for existing Zephyr

   configuration options, refer to the :makevar:`CONF_FILE` description above.

   For example, if you have a file named :file:`Kconfig` in the same directory

   as your application's :file:`CMakeLists.txt`, add the following line: