Improve command-line help text for `--liquidity-baking-toggle-vote`

The help text for --liquidity-baking-toggle-vote is confusing for some. See this Tezos Agora post: https://forum.tezosagora.org/t/triplespeak/4718

Consider updating the help text, e.g. to what the user suggested:

The baker has three options for this flag: Off to request ending the subsidy, On to request continuing or restarting the subsidy, and Pass to abstain.