docs: vmcoreinfo: add lockless printk ringbuffer vmcoreinfo

With the introduction of the lockless printk ringbuffer, the
VMCOREINFO relating to the kernel log buffer was changed. Update the
documentation to match those changes.

Fixes: 896fbe20

 ("printk: use the lockless ringbuffer")
Reported-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: John Ogness <john.ogness@linutronix.de>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Signed-off-by: Petr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/20200814213316.6394-1-john.ogness@linutronix.de