ARROW-13397: [R] Update arrow.Rmd vignette#11202

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thisisnic wants to merge 3 commits intoapache:masterfrom

thisisnic:ARROW-13397_arrow.Rmd

Commits on Sep 21, 2021

Re-order conversion tables, restart numbering, add explanation of float64 == double
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Commits on Oct 4, 2021

Remove extraneous reference
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Commits on Oct 6, 2021

add "currently"
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