Constant EXTRA_KCONFIG_HELP 

pub const EXTRA_KCONFIG_HELP: &str = "Additional kconfig fragment merged on top of \
     the baked-in `ktstr.kconfig`. Same line shapes the kernel uses: \
     `CONFIG_FOO=y`, `CONFIG_FOO=m`, `CONFIG_FOO=\"value\"`, and \
     `# CONFIG_FOO is not set`. User values win on conflict; \
     `make olddefconfig` resolves dependencies. Each unique fragment \
     produces a distinct cache slot via the `kc{baked}-xkc{extra}` \
     key suffix. After build, `validate_kernel_config` rejects \
     entries that disabled critical baked-in symbols \
     (CONFIG_SCHED_CLASS_EXT, CONFIG_DEBUG_INFO_BTF, CONFIG_BPF_SYSCALL, \
     CONFIG_FTRACE, CONFIG_KPROBE_EVENTS, CONFIG_BPF_EVENTS). \
     The baked-in fragment lives at `ktstr.kconfig` in the ktstr \
     repository root.";

Short clap-help for --extra-kconfig. Mirrors the CPU_CAP_HELP pattern: terse first sentence on the clap surface, full rustdoc on the KernelCommand::Build variant for --help long-about.

The full rustdoc covers: accepted line shapes, kbuild last-wins rule, make olddefconfig dependency resolution, post-build validate_kernel_config interaction, two-segment cache key, override warnings, and the unaddressable-from-other-flags rationale.