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MINOR: [R][Docs] Add note about conversion from JSON types to Arrow types (#13871)

Add note about conversion from JSON types to Arrow types.
These documents were copied from `docs/source/python/json.rst` with modifications.

Also, show the data frame in the example to make it easier to understand how the conversion is performed.

Authored-by: SHIMA Tatsuya <ts1s1andn@gmail.com>
Signed-off-by: Neal Richardson <neal.p.richardson@gmail.com>