Documentation/admin-guide: Add bus lock ratelimit

Since bus lock rate limit changes the split_lock_detect parameter,
update the documentation for the change.

Signed-off-by: Fenghua Yu <fenghua.yu@intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Tony Luck <tony.luck@intel.com>
Link: https://lore.kernel.org/r/20210419214958.4035512-4-fenghua.yu@intel.com