[SPARK-16142][R] group naiveBayes method docs in a single Rd

## What changes were proposed in this pull request?

This PR groups `spark.naiveBayes`, `summary(NB)`, `predict(NB)`, and `write.ml(NB)` into a single Rd.

## How was this patch tested?

Manually checked generated HTML doc. See attached screenshots.

![screen shot 2016-06-23 at 2 11 00 pm](https://cloud.githubusercontent.com/assets/829644/16320452/a5885e92-394c-11e6-994f-2ab5cddad86f.png)

![screen shot 2016-06-23 at 2 11 15 pm](https://cloud.githubusercontent.com/assets/829644/16320455/aad1f6d8-394c-11e6-8ef4-13bee989f52f.png)

Author: Xiangrui Meng <meng@databricks.com>

Closes #13877 from mengxr/SPARK-16142.