docs: enhance validation guide with tip on preventing submission when form is untouched

[kostyniuk]

🎯 Changes

Add guidance for preventing form submission when no fields are modified. Clarify correct approach and improve developer understanding.

✅ Checklist

• I have followed the steps in the Contributing guide.
• I have tested this code locally with pnpm test:pr.

🚀 Release Impact

• This change affects published code, and I have generated a changeset.
• This change is docs/CI/dev-only (no release).

[changeset-bot]

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Latest commit: 591cd80

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[LeCarbonator]

This isn't react-specific, so I'll copy the lines to the other framework-specific guides too. Do you mind if I commit them to this PR directly?

[kostyniuk]

@LeCarbonator let's do this, thanks