clap_man doesn't show the right number of value names

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• I have searched the discussions
• I have searched the existing issues

Rust Version

rustc 1.57.0 (f1edd0429 2021-11-29)

Clap Version

3.0.0-rc.0

Minimal reproducible code

TBD

Steps to reproduce the bug with the above code

TBD

Actual Behaviour

When --help renders value_names, i think it repeats the last one until we've hit the max number of values

Expected Behaviour

We literally show the value names as returned by the Arg

Additional Context

Split out of #3174

Debug Output

No response