[Feature] Generate parameter description with possbile values.

[BethanyZhou]

If we have a parameter with PSArgumentCompleter('A', 'B') or ArgumentCompleter('A', 'B'), it's nice to list its possible values in its parameter description in help doc.

[isra-fel]

The way argument completer is decribed in help doc should be different from validate set. We should avoid giving user the empression this is all the options.

[isra-fel]

Nice to have: if swagger description already has possible values, it's best we don't say it again.