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

Documentation/admin-guide/sysctl/kernel.rst

@@ -1544,6 +1544,13 @@ constant ``FUTEX_TID_MASK`` (0x3fffffff).
 If a value outside of this range is written to ``threads-max`` an
 ``EINVAL`` error occurs.
 
+timer_migration
+===============
+
+When set to a non-zero value, attempt to migrate timers away from idle cpus to
+allow them to remain in low power states longer.
+
+Default is set (1).
 
 traceoff_on_warning
 ===================