Clarify meaning of "set flag" in cmdstan-guide

[katrinabrock]

Submission Checklist

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

Summary

State explicitly that setting STAN_THREADS, STAN_MPI, and STAN_OPENCL to any value, including falsy values, turns the corresponding feature on.

Fix stan-dev/cmdstan#1293

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):
Max Planck Institute of Animal Behavior

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]

I think "defined" is slightly more correct than "set", but this is already an improvement so I would be happy to merge it

[katrinabrock]

@WardBrian done

[WardBrian]

Thanks!