Add variable description to fishing example

[cstjean]

I had to ask an LLM for help, so I figured that others would find this useful. Please feel free to edit for better prose/accuracy.

As an aside, when I read the word "fishing rate", I expected it to designate number_of_fish_caught / x, instead of number_of_fish_caught / Umax, so perhaps explaining that would be useful.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.77%. Comparing base (6df0b53) to head (54bfff1).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #418   +/-   ##
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  Coverage   99.77%   99.77%           
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  Files          38       38           
  Lines        6586     6586           
=======================================
  Hits         6571     6571           
  Misses         15       15           

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[pulsipher]

Thanks for the PR!