[CLI] document `defaultThemeVariants`

[brian-smith-tcril]

The help text is currently

paragon/bin/paragon-scripts.js

Lines 182 to 189 in cce7beb

	{
	name: '--defaultThemeVariants',
	description: `Specifies default theme variants. Defaults to a single 'light' theme variant.
	You can provide multiple default theme variants by passing multiple values, for
	example: \`--defaultThemeVariants light dark\`
	`,
	defaultValue: 'light',
	},

This was described in a forum post as

This gives no indication about what it actually does, or why I would desire this, or not

For this we should:

• Make the help text answer base-level questions
• Possibly write more detailed documentation and link to it from the help text