Update section on stanc for new --color flag, warning formatting

[WardBrian]

Submission Checklist

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

Summary

Some accumulated updates for stanc3's users guide chapter

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• Code: BSD 3-clause (https://opensource.org/licenses/BSD-3-Clause)
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[bob-carpenter]

Thanks for revising the examples. I don't know if they're complete and didn't check you reproduced them exactly, but the new output messages look good.