Low hanging fruit improvements in usage messages

From !228 (merged):

For sigsum-monitor, it would make sense with a "BUGS" section saying that it is work in progress and currently ignoring cosignatures.

And see also could also reference doc/monitor.md in some way (not sure if it should just be a local filename for user to figure out, or gitlab url. When package, would make sense to refer to location under /usr/share/doc).

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Also check if we should have descriptions for sigsum-key. Also check if we can expand a few of the command-line option descriptions which are a bit scarce.

Maybe add a missing short option or two.