ceph: document the `deviceClass` setting for OSDs
ceph: document the `deviceClass` setting for OSDs

This functionality was merged in with #3696, but not documented.

Signed-off-by: Stuart P. Bentley <a0f1490a20d0211c997b44bc357e1972deab8ae3@stuartpb.com>

rook/nfs,rook/rook,rook/rook,rook/nfs,rook/rook,rook/nfs,rook/rook,rook/rook