Documentation/sysctl: Add timer_migration to kernel.rst

There is no mention of timer_migration in the docs. Add
a short description.

Signed-off-by: Phil Auld <pauld@redhat.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: linux-doc@vger.kernel.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20250114190525.169022-1-pauld@redhat.com