LVM support in manpage

[Karl-Philipp Richter]

The manpage of 1.3.1 contains explanations of some lvm commands (explanations refer to LVM as well), e.g. `linux_lvm_cmd_lvcreate`, but doesn't mention what LVM support rsnapshot acutally refers to (Does rsnapshot create LVM snapshots automatically to read from them (instead of the original filesystem)? If yes, how is that activated (by default, an argument) and prevented (or is the whole rsnapshot based on LVM snapshooting and does never simply read from the filesystem)?).