Added information variables used to the soil model implementation docs

[jacobcook1995]

Description

Basically just the title. There's a big TODO about writing docs to explain the implementation properly at some point, but that can wait.

Fixes #883

Type of change

• New feature (non-breaking change which adds functionality)
• Optimization (back-end change that speeds up the code)
• Bug fix (non-breaking change which fixes an issue)

Key checklist

• Make sure you've run the pre-commit checks: $ pre-commit run -a
• All tests pass: $ poetry run pytest

Further checks

• Code is commented, particularly in hard-to-understand areas
• Tests added that prove fix is effective or that feature works
• Relevant documentation reviewed and updated

[vgro]

LGTM

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.80%. Comparing base (5e3ceef) to head (d3b7028).
⚠️ Report is 1987 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #884   +/-   ##
========================================
  Coverage    94.80%   94.80%           
========================================
  Files           75       75           
  Lines         5597     5597           
========================================
  Hits          5306     5306           
  Misses         291      291           

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