[R] Use roxygen @examplesIf tag in R docs

[asfimport]

The development version of roxygen2 supports a new tag @examplesIf to include examples conditionally: r-lib/roxygen2@513c9e2

Use this to remove the feature availability conditionals that are used in some examples, including:

• if (arrow_available()) \{...\}

• if (arrow_with_dataset()) \{...\}

• if (arrow_with_parquet()) \{...\}

• if (arrow_with_s3()) \{...\}

  This will simplify the examples and avoid giving the misleading suggestion that users need to perform these checks this every time.

Reporter: Ian Cook / @ianmcook
Assignee: Nicola Crane / @thisisnic

Related issues:

• [R] Allow Parquet and Arrow Dataset to be optional components (relates to)

PRs and other links:

• GitHub Pull Request #10373

_{Note: This issue was originally created as ARROW-11849. Please see the migration documentation for further details.}

[asfimport]

Ian Cook / @ianmcook:
Update based on continued work in ARROW-11735:
The arrow_with_dataset() and arrow_with_parquet() conditionals have been removed from all the examples, and examples that contained them have been wrapped in \dontrun instead of \donttest which gives us a stronger guarantee that these examples will not cause check failures. After using @examplesIf, switch these examples back to using \donttest, or use neither if that works.

[asfimport]

Ian Cook / @ianmcook:
Issue resolved by pull request 10373
#10373