add beta_neg_binomail docs

[lingium]

Submission Checklist

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

Summary

Add documentation for function beta_neg_binomial_lccdf. See stan-dev/math#3114.

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): Zhi Ling

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/)

[WardBrian]

Hi @lingium - since it looks like they’re all going to make it into this release, do you want to go ahead and add the lcdf, cdf, and RNG signatures in this PR?

[lingium]

Hi @WardBrian , yes, let me add them all here. BTW when will be the next release?

[WardBrian]

The release candidates will go out next week, and unless any issues get discovered the release will be 12/9

[WardBrian]

Thanks!