feat(shortint_integer): added documentation

[Anas-B]

Description

fixes https://github.com/zama-ai/concrete_internal/issues/543

Checklist

• [x] Tests for the changes have been added (for bug fixes / features)
• [x] Docs have been added / updated (for bug fixes / features)
• [x] The PR description links to the related issue (to link an issue, use '#XXX'.)
• [ ] The tests on AWS have been launched and are successful (apply the aws_test to the PR to launch the tests on AWS)

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[tmontaigu]

@cla-bot check

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