doc: add docs for rbd_default_order

doc: add docs for rbd_default_order

Adds documentation how to change default rbd object size. With the
previous option `--order` it was easy to guess the config name for the
default value, with the current option name `--object-size` thats hard
to guess.

Also extends the documentation for rbd_default_order to include
* how object-size is derived from the configured value
* allowed range of the value

In the first version of this commit I also added min and max for this
parameter (12/25, matching the object size range in `man 8
rbd`/Striping/object-size), but this made some tests fail, since some
seem to set values outside this range (and probably are fine since
included for some time already). To have this a doc-change only, I
removed the range.

Signed-off-by: Mara Sophie Grosch <littlefox@lf-net.org>