ARROW-15193: [R][Documentation] Update R binding documentation

[paleolimbot]

Now that #11904 is merged (ARROW-15010), we have slightly different syntax for defining bindings between compute C++ and R functions. @thisisnic wrote some excellent documentation for creating bindings which needs a few tweaks to reflect how this is now done!

[github-actions]

https://issues.apache.org/jira/browse/ARROW-15193

[github-actions]

⚠️ Ticket has not been started in JIRA, please click 'Start Progress'.

[thisisnic]

Thanks for updating these docs @paleolimbot, this looks great!

[ursabot]

Benchmark runs are scheduled for baseline = b596f29 and contender = 08096d4. 08096d4 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 ⬇️0.0% ⬆️0.0%] ursa-i9-9960x
[Finished ⬇️0.62% ⬆️0.0%] ursa-thinkcentre-m75q
Supported benchmarks:
ec2-t3-xlarge-us-east-2: Supported benchmark langs: Python. Runs only benchmarks with cloud = True
ursa-i9-9960x: Supported benchmark langs: Python, R, JavaScript
ursa-thinkcentre-m75q: Supported benchmark langs: C++, Java