machine: `machine list` Make name arguments optional and show all machines when it was not provided.

[karajan1001]

@karajan1001 Another minor UI tweak is that we need to make clear that the positional arguments are optional and all machines will be shown by default.

Originally posted by @dberenbaum in #6602 (comment)

[karajan1001]

@dberenbaum, Look into the code and the test on my local machine, I found that it had already been supported. And we had already had a test of it.

https://github.com/iterative/dvc/blob/e2d705fd76771c184f85a477f8d73aea40a96cab/tests/func/machine/test_machine_config.py#L95-L102

[dberenbaum]

@karajan1001 Thanks, but I meant here that we need to document that behavior in the help message.

Compare to dvc stage list --help output:

$ dvc stage list --help
usage: dvc stage list [-h] [-q | -v] [--all] [--fail] [-R] [--names-only]
                      [targets ...]

List stages.
Documentation: <https://man.dvc.org/stage/list>

positional arguments:
  targets          Show stages from a dvc.yaml/.dvc file or a directory. 'dvc.yaml' by
                   default