[R] Documentation for `read_parquet()` et al needs updating

[thisisnic]

Describe the bug, including details regarding any error messages, version, and platform.

#35173 results in the read_*() family of functions now always returning tibbles, but the docs still say data.frame() - we should also cherry-pick the fix for this into 13.0.0 and make sure it's reflected on the deployed Arrow site

Component(s)

R

[assignUser]

I have added the breaking change label to #35173 and added the 13 milestone to make sure it get's added to the next rc
cc @raulcd

[thisisnic]

Thanks very much @assignUser !