アクセサマクロのロールの'names:'の説明を修正

[stzn]

Closes #648

[Copilot]

Pull Request Overview

This PR corrects the documentation about the names: argument requirement for different macro roles in Swift. The change clarifies when accessor macros need to specify the names: argument and explains the restrictions for property observer generation.

Key Changes

• Clarified that peer and member macro roles require the names: argument
• Specified that accessor macros only need names: when generating willSet or didSet property observers
• Added explanation that accessor macros generating property observers cannot add other accessors

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