Documentation: bootconfig: document build-time cmdline rendering

Add a section describing CONFIG_CMDLINE_FROM_BOOTCONFIG: what it
does (renders the embedded "kernel" subtree to a flat cmdline at
build time so early_param() handlers see the values), what it
requires (BOOT_CONFIG_EMBED, a non-empty BOOT_CONFIG_EMBED_FILE,
CONFIG_CMDLINE to be empty, and ARCH_SUPPORTS_CMDLINE_FROM_BOOTCONFIG --
currently x86 only), the bootconfig opt-in semantics, the initrd-vs-embedded
precedence, and the soft-error overflow behavior.

This addresses feedback from the Sashiko AI review and Masami Hiramatsu to
document the CONFIG_CMDLINE requirement, which is enforced at the Kconfig
level but was not mentioned in the documentation, potentially confusing users
who might satisfy all other requirements but still find the option hidden in
menuconfig if CONFIG_CMDLINE is non-empty.

Link: https://lore.kernel.org/all/20260626-bootconfig_using_tools-v7-6-24ab72139c29@debian.org/

Signed-off-by: Breno Leitao <leitao@debian.org>
Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>

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