some deprecations

[nhuurre]

Submission Checklist

• [X] Run unit tests
• Documentation
  • [X] If a user-facing facing change was made, the documentation PR is here: stan-dev/docs#563
  • [ ] OR, no user-facing changes were made

Closes #494 and touches on #823 . Those issues have not received any feedback, not sure if it's because everyone agrees or because no one reads the issue tracker.

Release notes

• Deprecate using real scalar as a boolean
• Warn about ambiguous comparison operator chains

Copyright and Licensing

Copyright holder: Niko Huurre

By submitting this pull request, the copyright holder is agreeing to license the submitted work under the BSD 3-clause license (https://opensource.org/licenses/BSD-3-Clause)

[WardBrian]

I think this is good to go. I was going to ask why you weren't warning inside of the TernaryIf expression, but it turns out that already requires ints for the conditionals. The more you know.

Would you mind opening a docs PR to add the deprecation of reals as bools to this page? We might also want to update the warnings guide but that is optional, IMO