rest: add note on content type

[Samk13]

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Description

• Highlighting the introduction of a new content type to make it clearer and reduce any potential confusion for users.

Checklist

Ticks in all boxes and 🟢 on all GitHub actions status checks are required to merge:

• [ ] I'm aware of the code of conduct.
• [ ] I've created logical separate commits and followed the commit message format.
• [ ] I've targeted the master branch.
• [ ] If this documentation change impacts the current release of InvenioRDM, I will backport it to the production branch following approval or indicate to a maintainer that it should be backported.

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