docs: preset-of-presets description is skipped in onboarding

[jvmdc]

Changes

When a preset consisting only of presets is part of the onboarding configuration, its description is not included in the configuration summary. I could not find any documentation of this, so I have added this information to the configuration field.

The code of the behaviour being documented is here:

renovate/lib/config/presets/index.ts

Lines 283 to 290 in 1ff5cd3

	if (
	presetKeys.length === 2 &&
	presetKeys.includes('description') &&
	presetKeys.includes('extends')
	) {
	// preset is just a collection of other presets
	delete presetConfig.description;
	}

Context

#31500

Documentation (please check one with an [x])

• I have updated the documentation, or
• No documentation update is required

How I've tested my work (please select one)

I have verified these changes via:

• Code inspection only, or
• Newly added/modified unit tests, or
• No unit tests but ran on a real repository, or
• Both unit tests + ran on a real repository

[renovate-release]

🎉 This PR is included in version 38.93.6 🎉

The release is available on:

• GitHub release
• npm package (@latest dist-tag)
• 38.93.6

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