Explain how to access HD when it's a LVM volume

Originally created by @BitingBird on #8881 (Redmine)

doc/encryption_and_privacy/your_data_wont_be_saved_unless_explicitly_asked.mdwn -> the note about LVM being “more complicated” could be explained.

Pertinent discussion from #5636 (closed):

Regarding LVM, what happens by default?

-> Nothing useful when a PV appears (e.g. after unlocking a dm-crypt volume that hosts a PV).

what happens if you run vgchange -ay?

-> Then the VG and all LVs in there appear. Maybe they can even be mounted from GNOME Disks (not sure).

Related issues

• Related to #8949 (closed)
• Blocked by #5636 (closed)