Add a note regarding element containment testing

[user202729]

Something I notice while working on #39717.
Previously, the clause "Otherwise, …" doesn't hold.

📝 Checklist

• The title is concise and informative.
• The description explains in detail what this PR is about.
• I have linked a relevant issue or discussion.
• I have created tests covering the changes.
• I have updated the documentation and checked the documentation preview. (see https://doc-pr-39718--sagemath.netlify.app/html/en/thematic_tutorials/coercion_and_categories.html#equality-and-element-containment)

⌛ Dependencies

[github-actions]

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