cli: the `registry add` command help doesn't detail registry aliasing

Apart from the usage, the example do not describe to two positional arguments on the registry add command. It would be useful to expand this help output, so the alias and source are explained to the user.

Start at the `registry add` command's help or usage entry point and inspect how its two positional arguments are currently described. Update the help text so the registry alias and source are explained, then run the command's help output to confirm both arguments are clear.

Written by the indexing model from the issue text.