Resolve issues building users guide with Ubuntu 20.04/Doxygen 1.8.17

Checklist (Please check before submitting)

[x] I reviewed the Contributing Guide.
[x] I reviewed the CF README.md file to see if the feature is in the major future work.
[x] I performed a cursory search to see if the feature request is relevant, not redundant, nor in conflict with other tickets.

Is your feature request related to a problem? Please describe. Doxygen warnings for documented empty return type

Describe the solution you'd like Remove unnecessary documentation

Describe alternatives you've considered None

Additional context None

Requester Info Jacob Hageman - NASA/GSFC

nasa / FM