Update tuple doc to allow for length one tuples

[WardBrian]

Submission Checklist

• Builds locally
• New functions marked with <<{ since VERSION }>>
• Declare copyright holder and open-source license: see below

Summary

Doc for stan-dev/stanc3#1609

Copyright and Licensing

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Simons Foundation

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• Code: BSD 3-clause (https://opensource.org/licenses/BSD-3-Clause)
• Documentation: CC BY-ND 4.0 (https://creativecommons.org/licenses/by-nd/4.0/)