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 <[e-mail]>
Signed-off-by: Gang Yan <[e-mail]>
Reviewed-by: David Disseldorp <[e-mail]>
Reviewed-by: Nicolas Schier <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Nicolas Schier <[e-mail]>

Documentation/kbuild/kbuild.rst

···
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 Setting this to a date string overrides the timestamp used in the
 UTS_VERSION definition (uname -v in the running kernel). The value has to
-be a string that can be passed to date -d. The default value
-is the output of the date command at one point during build.
+be a string that can be passed to date -d. E.g.::
+
+    $ KBUILD_BUILD_TIMESTAMP="Mon Oct 13 00:00:00 UTC 2025" make
+
+The default value is the output of the date command at one point during
+build. If provided, this timestamp will also be used for mtime fields
+within any initramfs archive. Initramfs mtimes are 32-bit, so dates before
+the 1970 Unix epoch, or after 2106-02-07 06:28:15 UTC will fail.
 
 KBUILD_BUILD_USER, KBUILD_BUILD_HOST
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