doc: update params of memhp_default_state=

Commit 5f47adf7 ("mm/memory_hotplug: allow to specify a default
online_type") allows to specify a default online_type which make
online memory to kernel or movable zone possible but fail to update
to doc. Update doc to fit this change.

Signed-off-by: Ma Wupeng <mawupeng1@huawei.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20230802074312.2111074-1-mawupeng1@huawei.com