man: lvmthin should use clearer thin pool options

man: lvmthin should use clearer thin pool options

Previously, this was the recommended form for creating a thin pool:

lvconvert --thinpool VG/ThinDataLV --poolmetadata VG/ThinMetaLV

but this is confusing, because --thinpool does not actually take
an arg, and is more naturally used to specify an existing thin pool.

The new recommended form is:

lvconvert --type thin-pool --poolmetadata VG/ThinMetaLV VG/ThinDataLV