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ARROW-6811: [R] Assorted post-0.15 release cleanups

Various minor cleanups and typos I addressed while building 0.15 for CRAN and updating the docs. Also include some upstream changes to the autobrew scripts.

Closes #5601 from nealrichardson/post-0.15 and squashes the following commits:

aebb8412d <Neal Richardson> Various cleanups and updates to R things after 0.15

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