added documentation for the Dirichlet-multinomial distribution

[chvandorp]

Submission Checklist

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

Summary

In a recent PR I added the Dirichlet-multinomial distribution to the Stan math library. This PR is for adding documentation to the functions reference. I modified the section on discrete multivariate distributions. I wasn't sure what version number to use. Based on other PRs, I guessed it should be 2.34.

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): Christiaan H. van Dorp

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/)

[andrjohns]

Thanks!