kbuild: doc: improve KBUILD_BUILD_TIMESTAMP documentation

kbuild: doc: improve KBUILD_BUILD_TIMESTAMP documentation

This patch adds an example of how to set KBUILD_BUILD_TIMESTAMP to a
specific date. Also, note that the provided timestamp is used for
initramfs mtime fields, which are 32-bit and thus limited to dates
between the Unix epoch and 2106-02-07 06:28:15 UTC. Dates outside this
range will cause errors.

Suggested-by: David Disseldorp <ddiss@suse.de>
Signed-off-by: Gang Yan <yangang@kylinos.cn>
Reviewed-by: David Disseldorp <ddiss@suse.de>
Reviewed-by: Nicolas Schier <nsc@kernel.org>
Link: https://patch.msgid.link/20251017021209.6586-1-gang.yan@linux.dev
Signed-off-by: Nicolas Schier <nsc@kernel.org>