ARROW-17690: [R] Implement dplyr::across() inside distinct()

[nealrichardson]

The feature was actually done by ARROW-17689; this PR just adds the test and updates the docs.

cc @eitsupi

[github-actions]

https://issues.apache.org/jira/browse/ARROW-17690

[ursabot]

Benchmark runs are scheduled for baseline = 4f31bfc and contender = 2577ac1. 2577ac1 is a master commit associated with this PR. Results will be available as each benchmark for each run completes.
Conbench compare runs links:
[Finished ⬇️0.0% ⬆️0.0%] ec2-t3-xlarge-us-east-2
[Failed ⬇️0.14% ⬆️0.0%] test-mac-arm
[Failed ⬇️0.28% ⬆️0.0%] ursa-i9-9960x
[Finished ⬇️0.11% ⬆️0.04%] ursa-thinkcentre-m75q
Buildkite builds:
[Finished] 2577ac1a ec2-t3-xlarge-us-east-2
[Failed] 2577ac1a test-mac-arm
[Failed] 2577ac1a ursa-i9-9960x
[Finished] 2577ac1a ursa-thinkcentre-m75q
[Finished] 4f31bfc2 ec2-t3-xlarge-us-east-2
[Failed] 4f31bfc2 test-mac-arm
[Failed] 4f31bfc2 ursa-i9-9960x
[Finished] 4f31bfc2 ursa-thinkcentre-m75q
Supported benchmarks:
ec2-t3-xlarge-us-east-2: Supported benchmark langs: Python, R. Runs only benchmarks with cloud = True
test-mac-arm: Supported benchmark langs: C++, Python, R
ursa-i9-9960x: Supported benchmark langs: Python, R, JavaScript
ursa-thinkcentre-m75q: Supported benchmark langs: C++, Java

[ursabot]

['Python', 'R'] benchmarks have high level of regressions.
test-mac-arm