Update documentation for specific models

[athowes]

After we have the preprocessing interface solidified, we should go back and make sure that the models we have are documented well taking that preprocessing into account.

For example right now:

#' Prepare latent individual model
#'
#' @param data A `data.frame` containing line list data
#' @family latent_individual
#' @export
as_latent_individual <- function(data) {
  UseMethod("as_latent_individual")
}

I was going to replace data here with an @inheritParam from somehwere, but really this data should be a different kind of parameter. One that is before the processing. Perhaps it should say "an object created using as_linelist() (or another constructor).

Another example issue:

#' Define the model-specific component of an `epidist` custom formula
#'
#' @param data A `data.frame` containing line list data
#' @param formula As produced by [brms::brmsformula()]
#' @param ... ...
#' @method epidist_formula_model epidist_latent_individual
#' @family latent_individual
#' @export
epidist_formula_model.epidist_latent_individual <- function(
  data, formula, ...
) {
  # data is only used to dispatch on
  formula <- stats::update(
    formula, delay | vreal(relative_obs_time, pwindow, swindow) ~ .
  )
  return(formula)
}

This shouldn't say formula as produced by brms::brmsformula() since it needs to have been run through epidist_formula. How to do this and share the argument nicely?

We also need to do this for the direct / pass through model of PR #393.