[Developer Issue]: Documenting things using Rcpp or C++

[kellijohnson-NOAA]

We should document units alongside code but which method of doing that is unclear. Below are options and some questions

1. Should we document units alongside the C++ code and display them on the Doxygen website (e.g., updating the documentation near log_rzero to include the units
   https://github.com/NOAA-FIMS/FIMS/blob/main/inst/include/population_dynamics/recruitment/functors/recruitment_base.hpp#L42

2. Should we document units in the Rcpp interface following approach 4 listed here? For example, we can document units in the age comp data interface:

Rcpp::class_<AgeCompDataInterface>("AgeComp")
      .constructor<int, int>()
      .field("age_comp_data",
              &AgeCompDataInterface::age_comp_data,
              "Age composition data can be in either numbers or proportions")
      .method("get_id",
              &AgeCompDataInterface::get_id,
              "Get the ID of the age composition interface");

Then user can view the documentation in R

> FIMS::AgeComp
C++ class 'AgeComp' <000001cdcbc66d20>
Constructors:
    AgeComp(int, int)

Fields: 
    Rcpp::Vector<14, Rcpp::PreserveStorage> age_comp_data
        docstring : Age composition data can be in either numbers or proportions

Methods: 
     unsigned int get_id()  
           docstring : Get the ID of the age composition interface

3. Can we use TMB for documenting them? For example, by modifying reporting section in model.hpp?

Originally posted by @Bai-Li-NOAA in #561 (comment)