[R][Documentation] Update R binding documentation

[asfimport]

After ARROW-15010, the syntax for defining bindings will have changed. We'll need to update the documentation to reflect this.

Reporter: Dewey Dunnington / @paleolimbot
Assignee: Dewey Dunnington / @paleolimbot

Related issues:

• [R][Documentation] Document the process of creating R bindings for compute kernels and rationale behind conventions (relates to)
• [R] Create a function registry for our NSE funcs (relates to)

PRs and other links:

• GitHub Pull Request #12075

_{Note: This issue was originally created as ARROW-15193. Please see the migration documentation for further details.}

[asfimport]

Nicola Crane / @thisisnic:
Issue resolved by pull request 12075
#12075