Document and test behavior of dbc search without --pre when driver is already installed #327
Description
I was momentarily confused today when I ran dbc search without --pre and it showed the ClickHouse driver, since that driver is only available as a pre-release.
Turns out the reason it was showing it is that I had already installed it, and dbc search will always list installed drivers regardless of pre-release status.
I think this behavior makes sense, but it's not documented anywhere, and I'm not sure if it's tested. We should document it and add tests if there aren't tests already.
Documenting it could be as simple as changing (hidden by default) to (hidden by default unless already installed) in a few places.