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ARROW-7186: [R] Add inline comments to document the dplyr code

In addition to adding comments to help explain how the machinery works, this PR also improves the messaging when a user tries to filter Arrow data with an expression that isn't yet supported, and it raises an error when that happens on a Dataset instead of a warning, under the assumption that you want to explicitly control when to pull data from a big multi-file dataset. (https://issues.apache.org/jira/browse/ARROW-7095 is a followup to be more clever about unsupported expressions.)

Closes #5847 from nealrichardson/dplyr-inline-docs and squashes the following commits:

42a7371bb <Neal Richardson> Apply any good filters before collecting and applying bad ones
236fc7605 <Neal Richardson> Better error message when filter expression isn't supported
8d1420227 <Neal Richardson> Add a bunch of comments to dplyr.R

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