dm thin: add data block size limits to Documentation

Inform users that the data block size can't be any arbitrary number,
i.e. its value must be between 64KB and 1GB.  Also, it should be a
multiple of 64KB.

Signed-off-by: Carlos Maiolino <cmaiolino@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>
Acked-by: Joe Thornber <ejt@redhat.com>