documentation: Document RCU self test boot params

Document the RCU self test boot parameters in kernel-parameters.txt.
Signed-off-by: NPranith Kumar <bobby.prani@gmail.com>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

Documentation/kernel-parameters.txt

@@ -3071,6 +3071,15 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
 			messages.  Disable with a value less than or equal
 			to zero.
 
+	rcupdate.rcu_self_test= [KNL]
+			Run the RCU early boot self tests
+
+	rcupdate.rcu_self_test_bh= [KNL]
+			Run the RCU bh early boot self tests
+
+	rcupdate.rcu_self_test_sched= [KNL]
+			Run the RCU sched early boot self tests
+
 	rdinit=		[KNL]
 			Format: <full_path>
 			Run specified binary instead of /init from the ramdisk,