doc: rcu: Suspicious RCU usage is a warning

Suspicious RCU usage messages are reported as warnings.

Fixes: a5dd63ef ("lockdep: Use "WARNING" tag on lockdep splats")
Signed-off-by: NGeert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: NPaul E. McKenney <paulmck@linux.ibm.com>
Signed-off-by: NJonathan Corbet <corbet@lwn.net>

Documentation/RCU/lockdep-splat.txt

@@ -14,9 +14,9 @@ being the real world and all that.
 So let's look at an example RCU lockdep splat from 3.0-rc5, one that
 has long since been fixed:
 
-===============================
-[ INFO: suspicious RCU usage. ]
--------------------------------
+=============================
+WARNING: suspicious RCU usage
+-----------------------------
 block/cfq-iosched.c:2776 suspicious rcu_dereference_protected() usage!
 
 other info that might help us debug this: