1443 update rainfall generator documentation

[vgro]

This PR updates the rainfall generator part of the documentation following #1444.

The docs are built here.

Fixes #1443

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

[jacobcook1995]

LGTM!

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.97%. Comparing base (dec416d) to head (8bf3b04).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #1451   +/-   ##
========================================
  Coverage    94.97%   94.97%           
========================================
  Files           71       71           
  Lines         7498     7498           
========================================
  Hits          7121     7121           
  Misses         377      377           

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