[SPARKR][DOC] minor formatting and output cleanup for R vignettes

## What changes were proposed in this pull request?

Clean up output, format table, truncate long example output, hide warnings

(new - Left; existing - Right)
![image](https://cloud.githubusercontent.com/assets/8969467/19064018/5dcde4d0-89bc-11e6-857b-052df3f52a4e.png)

![image](https://cloud.githubusercontent.com/assets/8969467/19064034/6db09956-89bc-11e6-8e43-232d5c3fe5e6.png)

![image](https://cloud.githubusercontent.com/assets/8969467/19064058/88f09590-89bc-11e6-9993-61639e29dfdd.png)

![image](https://cloud.githubusercontent.com/assets/8969467/19064066/95ccbf64-89bc-11e6-877f-45af03ddcadc.png)

![image](https://cloud.githubusercontent.com/assets/8969467/19064082/a8445404-89bc-11e6-8532-26d8bc9b206f.png)

## How was this patch tested?

Run create-doc.sh manually

Author: Felix Cheung <felixcheung_m@hotmail.com>

Closes #15340 from felixcheung/vignettes.