Docs for fatal_error statement

[WardBrian]

Submission Checklist

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

Summary

Docs for https://github.com/stan-dev/stanc3/pull/1402

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):

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]

I think some of the recent merges may have caused conflicts here. Happy to review when they're resolved.

[WardBrian]

@bob-carpenter I went ahead and rebased this to get rid of those conflicts