Reapply comment in async_get_or_create in device registry

[arturpragacz]

Proposed change

Reapply comment in async_get_or_create in device registry.

Was supposed to be in #179005 (comment), but somehow was not? Not sure what happened there.

@MartinHjelmare

Type of change

• Dependency upgrade
• Bugfix (non-breaking change which fixes an issue)
• New integration (thank you!)
• New feature (which adds functionality to an existing integration)
• Deprecation (breaking change to happen in the future)
• Breaking change (fix/feature causing existing functionality to break)
• Code quality improvements to existing code or addition of tests

Additional information

• This PR fixes or closes issue: fixes #
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[Copilot]

Pull request overview

Restores the intended explanatory comment in device registry collision handling.

Changes:

• Clarifies why parentless devices cannot reuse child identifiers.
• No functional code changes.

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[MartinHjelmare]

Thanks!