arrow

MINOR: [R] Clarify read_json_arrow() docs

A quick PR to clarify `read_json_arrow()` docs I found confusing while benchmarking. Specifically, specifies the function

- is for ndjson (as opposed to say the many json formats to which pandas can write a dataframe)
- handles compression
- handles implicit and explicit nulls (was in the example, but not previously stated)

Open to changes, but do feel these docs need to at least explicitly say "ndjson" somewhere.

Closes #13133 from alistaire47/chore/read-json-docs

Lead-authored-by: Edward Visel <1693477+alistaire47@users.noreply.github.com>
Co-authored-by: Neal Richardson <neal.p.richardson@gmail.com>
Signed-off-by: Neal Richardson <neal.p.richardson@gmail.com>