Add documentation on tuple-returning special functions

[WardBrian]

Submission Checklist

• Builds locally
• New functions marked with `r since("VERSION")`
• Declare copyright holder and open-source license: see below

Summary

Docs for stan-dev/math#2931 and stan-dev/stanc3#1346

Copyright and Licensing

Please list the copyright holder for the work you are submitting (this will be you or your assignee, such as a university or company):

Simons Foundation

By submitting this pull request, the copyright holder is agreeing to license the submitted work under the following licenses:

• 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/)

[bob-carpenter]

This looks good and I can be convinced that we don't want to duplicate what the function does, but the shape of returns should be indicated, I think. At least it should be consistent.

[bob-carpenter]

thanks!