driver core: add missing kernel-doc for union members

driver core: add missing kernel-doc for union members

The use of __SYSFS_FUNCTION_ALTERNATIVE() adds an anonymous union (or
struct if CONFIG_CFI=y).*

Describe the additional struct/union members to avoid docs build
warnings.

Warning: include/linux/device.h:117 struct member 'show_const' not described in 'device_attribute'
Warning: include/linux/device.h:117 struct member 'store_const' not described in 'device_attribute'

*: kernel-doc ignores CONFIG_ symbols in source files; it is using the
first definition of __SYSFS_FUNCTION_ALTERNATIVE(), which is struct
instead of union.

Fixes: 434506b86a6c ("driver core: Allow the constification of device attributes")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Reviewed-by: Thomas Weißschuh <linux@weissschuh.net>
Link: https://patch.msgid.link/20260623190023.407781-1-rdunlap@infradead.org
Signed-off-by: Danilo Krummrich <dakr@kernel.org>