docs: block: Document support for read-only partition in cmdline part

Document support for read-only partition in cmdline partition for block
devices by appending "ro" after the (partition name).

Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>
Link: https://lore.kernel.org/r/20241002221306.4403-3-ansuelsmth@gmail.com


Signed-off-by: Jens Axboe <axboe@kernel.dk>