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

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

[codecov[bot]]

Codecov Report

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

Additional details and impacted files

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