Documentation/x86: Update split lock documentation

Documentation/x86: Update split lock documentation

commit b041b525dab9 ("x86/split_lock: Make life miserable for split
lockers") added a delay and serialization of split locks. Commit
727209376f49 ("x86/split_lock: Add sysctl to control the misery mode")
provided a sysctl to turn off the misery.

Update the split lock documentation to describe the current state of
the code.

Signed-off-by: Tony Luck <tony.luck@intel.com>
Link: https://lore.kernel.org/r/20230315225722.104607-1-tony.luck@intel.com
Signed-off-by: Jonathan Corbet <corbet@lwn.net>