[SPARK-20550][SPARKR] R wrapper for Dataset.alias

## What changes were proposed in this pull request?

- Add SparkR wrapper for `Dataset.alias`.
- Adjust roxygen annotations for `functions.alias` (including example usage).

## How was this patch tested?

Unit tests, `check_cran.sh`.

Author: zero323 <zero323@users.noreply.github.com>

Closes #17825 from zero323/SPARK-20550.