[R] Better document the CSV read/parse/convert options we can use with open_dataset()

When a user opens a CSV dataset using open_dataset, they can take advantage of a lot of different options which can be specified via `CsvReadOptions$create()` etc.

However, as they are passed in via the ellipses (`...`) argument, it's not particularly clear to users which arguments are supported or not. They are not documented in the `open_dataset()` docs, and further confused (see the code for `CsvFileFormat$create()` by the fact that we support a mix of Arrow and readr parameters (though users can't use both at the same time).

We should better document the arguments we do support.

**Reporter**: [Nicola Crane](https://issues.apache.org/jira/browse/ARROW-18354) / @thisisnic

_{**Note**: *This issue was originally created as [ARROW-18354](https://issues.apache.org/jira/browse/ARROW-18354). Please see the [migration documentation](https://github.com/apache/arrow/issues/14542) for further details.*}

Start with the R documentation for open_dataset(), then inspect CsvReadOptions$create() and CsvFileFormat$create() to identify the supported ellipsis arguments. Document the supported Arrow and readr options, including that they cannot be used together; done means users can determine which parameters open_dataset() accepts.

Written by the indexing model from the issue text.