Document built-in transformation functions

[WardBrian]

Submission Checklist

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

Summary

Documents the newly-exposed built in transformation functions from stan-dev/stanc3#1494

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]

At least you can take comfort in the fact that it took me as long to write this review as it will take you to make all the changes :-)

[WardBrian]

Thanks for the detailed comments @bob-carpenter -- I think I incorporated all of them

[bob-carpenter]

Thanks!