Expand the fetchvars documentation.

[kdorheim]

Describe development

Expand fetchvars documentation as requested in #712.

Type of development

• MINOR: some functionality is added but backward compatible

^ Just expanding upon documentation

Checklist

• Roxygen where applicable but also thorough and coherent inline documentation
• Unit tests when applicable
• Complies with the Hector code style guide
• The PR passes automated testing suite

[github-actions]

Differences in Hector outputs

Hello, this is leeyabot! 🤖

The current pull request's outputs do not differ from 3.0.0 (9aebf67).

[codecov-commenter]

Codecov Report

❗ No coverage uploaded for pull request base (dev@192c369). Click here to learn what that means.
The diff coverage is n/a.

❗ Current head efa0767 differs from pull request most recent head e108b96. Consider uploading reports for the commit e108b96 to get more accurate results

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[bpbond]

Looks good!