Add warning in docs about #723

[glatterf42]

Since we won't be able to merge #723 in time, this PR adds a warning in the docs to where PRICE_EMISSION is defined. This will allow us to postpone the fix for 3.8.1 in January.

I'm actually not sure: should we update the release notes?

How to review

• Read the diff and note that the CI checks all pass.
• Check the documentation built by the RTD check below and look at MESSAGE core formulation -> Variable Definitions -> Auxiliary Variables.

PR checklist

• Continuous integration checks all ✅
• [ ] Add or expand tests; coverage checks both ✅ Only docs
• Add, expand, or update documentation.
• Update release notes.

[codecov]

Codecov Report

Merging #766 (56ffb93) into main (2d7b3f5) will not change coverage.
The diff coverage is n/a.

Additional details and impacted files

@@          Coverage Diff          @@
##            main    #766   +/-   ##
=====================================
  Coverage   95.2%   95.2%           
=====================================
  Files         46      46           
  Lines       4312    4312           
=====================================
  Hits        4107    4107           
  Misses       205     205           

[khaeru]

Looks good! I don't think we need to add it to the release notes.