Docs: Clarify how reference and compared directories are processed

[adrien-berchet]

No description provided.

[Copilot]

Pull Request Overview

This PR improves the documentation for the dir-content-diff library by clarifying how reference and compared directories are processed during comparison operations. The changes make it more explicit that the order of parameters matters and that extraneous files in the compared directory are ignored.

• Updated section introduction to be more direct and actionable
• Added explicit explanation of reference vs compared directory behavior
• Added warning callout about parameter order importance
• Clarified that only files present in both directories are compared

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