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

[asfimport]

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 / @thisisnic

_{Note: This issue was originally created as ARROW-18354. Please see the migration documentation for further details.}