Documentation/sysctl: document max_rcu_stall_to_panic

commit dfe56404 ("rcu: Panic after fixed number of stalls")
introduced a new systctl but no accompanying documentation.

Add a simple entry to the documentation.
Signed-off-by: NJoel Savitz <jsavitz@redhat.com>
Acked-by: NPaul E. McKenney <paulmck@kernel.org>
Signed-off-by: NJonathan Corbet <corbet@lwn.net>

Documentation/admin-guide/sysctl/kernel.rst

@@ -783,6 +783,13 @@ is useful to define the root cause of RCU stalls using a vmcore.
 1 panic() after printing RCU stall messages.
 = ============================================================
 
+max_rcu_stall_to_panic
+======================
+
+When ``panic_on_rcu_stall`` is set to 1, this value determines the
+number of times that RCU can stall before panic() is called.
+
+When ``panic_on_rcu_stall`` is set to 0, this value is has no effect.
 
 perf_cpu_time_max_percent
 =========================