How should `--type` be passed in

[supakeen]

There's a mix of information on how --type should be passed in. One place says it can be passed in multiple times (--type qcow2 --type raw), and another says to use a string array (--type [qcow2,raw]).

Even if both are supported, I think it would be good to standardise on which is recommended in the docs to avoid confusion. Or mentioning both in the same sentence/section, saying "both are supported".

Originally posted by @p5 in https://github.com/osbuild/bootc-image-builder/pull/791#discussion_r1911688296