doc: document collocated DB LV split for ceph-volume lvm batch

This commits documents the ability to deploy OSDs where the block.db
LV is carved out of the same physical device as the data LV, without
requiring a dedicated DB disk.

This can be done by passing --block-db-size without --db-devices to
`ceph-volume lvm batch`. The feature was introduced in Squid as part
of tracker #69996.

Fixes: https://tracker.ceph.com/issues/69996

Signed-off-by: Guillaume Abrioux <gabrioux@ibm.com>