[lldb] Specify aguments of `image list`

Register positional argument details in `CommandObjectTargetModulesList`.

I recently learned that `image list` takes a module name, but the help info
does not indicate this. With this change, `help image list` will show that it
accepts zero or more module names.

This makes it easier to get info about specific modules, without having to
find/grep through the full image list.

Reviewed By: DavidSpickett

Differential Revision: https://reviews.llvm.org/D125154