RBD Documentation and Example fixes for --image-format

 - RBD Documentation, --image-format wrongly specified as --format in examples
 - RBD Documentation, better describe image format, to differentiate from --format

Reviewed-by: Josh Durgin <josh.durgin@inktank.com>
man: re-generate changed man pages

Signed-off-by: Josh Durgin <josh.durgin@inktank.com>