kconfig-language: improve menuconfig usage description

Improper menuconfig usage leads to empty menu entries.
zconfdump() is able to reveal some real-life examples:
- menuconfig VFIO_NOIOMMU
- menuconfig RESET_CONTROLLER
- menuconfig SND_ARM

To avoid future occurrences of those, improve the menuconfig syntax
description.

Signed-off-by: Eugeniu Rosca <rosca.eugeniu@gmail.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>