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