[R] Add examples to functions which don't have examples

[asfimport]

There are quite a few functions in the R package which don't have code examples.  Some of these won't necessarily need them, but it would be useful to document some of the remaining ones.

Functions without examples:

"arrow_info", "cast_options", "Codec", "cpu_count", "CsvReadOptions",
"CsvTableReader", "dataset_factory", "Dataset", "default_memory_pool",
"dictionary", "Expression", "FeatherReader", "FileInfo", "FileSelector",
"FileSystem", "flight_connect", "flight_get", "flight_put", "FragmentScanOptions",
"install_arrow", "install_pyarrow", "io_thread_count", "list_flights",
"map_batches", "MessageReader", "mmap_create", "mmap_open", "ParquetArrowReaderProperties",
"ParquetFileWriter", "ParquetWriterProperties", "Partitioning",
"read_ipc_stream", "read_message", "read_schema", "Scanner",
"write_dataset"

Reporter: Nicola Crane / @thisisnic

_{Note: This issue was originally created as ARROW-13383. Please see the migration documentation for further details.}

[asfimport]

Nicola Crane / @thisisnic:
Most remaining undocumented items are objects or functions which are used mostly internally, so closing this ticket