documentation for `eval_f` is not complete

When I run R CMD check I get these issues:
> ── R CMD check results ──────────────────────────────── funtools 2021.05.27 ────
Duration: 22.4s

> checking Rd \usage sections ... WARNING
Undocumented arguments in documentation object 'eval_f'
‘x’ ‘...’

> Functions with \usage entries need to have the appropriate \alias
entries, and all their arguments documented.
The \usage entries must correspond to syntactically valid R code.
See chapter ‘Writing R documentation files’ in the ‘Writing R
Extensions’ manual.

> checking Rd contents ... WARNING
Argument items with no description in Rd object 'eval_f':
‘env’

> checking Rd files ... NOTE
prepare_Rd: eval_f.Rd:12-14: Dropping empty section \value

The actual code for `eval_f` can be found [here](https://github.com/BAAQMD/funtools/blob/master/R/eval_f.R). I reproduce it below:
```
#' eval_f
#'
#' @param x
#' @param ...
#' @param env
#'
#' @importFrom rlang as_quosure eval_tidy
#'
#' @return
#' @export
eval_f <- function (x, ..., env = NULL) {
quosured <- rlang::as_quosure(x, env = env)
rlang::eval_tidy(quosured, ...)
}
```
In short, the parameters are _named_ but not _described_ in the documentation.

I don't have enough experience with rlang to properly describe these variables. But it would be nice if someone updated the documentation so the options are described.

Start with the roxygen comments and implementation in R/eval_f.R, then run R CMD check to reproduce the eval_f documentation warnings. Read the rlang behavior relevant to x, ..., and env before documenting their meanings and adding a useful return-value description; done means the undocumented-argument warnings and empty-value note are resolved.

Written by the indexing model from the issue text.