add warning about default test order causing problems with MPI

[kbthomp1]

Summary

This adds a warning in the documentation about the default (random) test order of execution possibly causing issues for MPI programs using Catch2 after version 3.9.1.

We recently update Catch2 from version 3.1.0 to the 3.15.1, and we were surprised that our tests started encountering MPI deadlock due the default test execution being changed to random. The fix is simple (use --lex or another non-default ordering option), but a google search was unhelpful. I hope that including this warning in the documentation helps improve the visibility of the fix.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.24%. Comparing base (0aeb818) to head (d531948).

Additional details and impacted files

@@            Coverage Diff             @@
##            devel    #3192      +/-   ##
==========================================
- Coverage   91.27%   91.24%   -0.03%     
==========================================
  Files         204      204              
  Lines        8973     8973              
==========================================
- Hits         8190     8187       -3     
- Misses        783      786       +3     

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