char: misc: improve testing Kconfig description

Describe that it tests the miscdevice API and include the usual disclaimer
about KUnit not being fit for production kernels.

While at it, also fix KUnit capitalization.

Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@igalia.com>
Link: https://lore.kernel.org/r/20250123123249.4081674-2-cascardo@igalia.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>