arrow

ARROW-15011: [R] Generate documentation for dplyr function bindings (#14014)

Approach:

* `register_binding` takes an additional optional argument, `notes`, where you can list any limitations or differences in behavior between the Arrow version and the R function
* These notes are put in the `.cache` environment when the nse_funcs are built.
* New script `data-raw/docgen.R` that reads `arrow:::.cache$docs` and writes out `dplyr-funcs-docs.R` containing roxygen.
* Similarly, we pull the dplyr functions we s3_register and add them to the generated docs. Unfortunately, the notes about feature limitations aren't easily kept alongside the functions themselves because of how they're registered on load. The approach here creates a list in `arrow-package.R`, where the `.onLoad()` happens, and notes go there. 
* Docs and crossreferences are generated by roxygen2 as usual. 

I deferred filling in all of the function notes. See followup JIRAs on ARROW-17665. 

Authored-by: Neal Richardson <neal.p.richardson@gmail.com>
Signed-off-by: Neal Richardson <neal.p.richardson@gmail.com>