Clarify that read_stan_csv does not work with recent CmdStanR CSVs

[jgabry]

We can't guarantee that read_stan_csv will work with CSV files generated by recent (or future) versions CmdStanR. This PR adds a note in the documentation to warn users about this. The function still works with files generated by RStan when sample_file is specified, so we shouldn't actually deprecate it, but users continue try to use it with CmdStanR CSVs so we should mention this in the doc.

closes https://github.com/stan-dev/rstan/issues/1133