[SPARK-18849][ML][SPARKR][DOC] vignettes final check update
[SPARK-18849][ML][SPARKR][DOC] vignettes final check update

## What changes were proposed in this pull request?

doc cleanup

## How was this patch tested?

~~vignettes is not building for me. I'm going to kick off a full clean build and try again and attach output here for review.~~
Output html here: https://felixcheung.github.io/sparkr-vignettes.html

Author: Felix Cheung <felixcheung_m@hotmail.com>

Closes #16286 from felixcheung/rvignettespass.

(cherry picked from commit 7d858bc5ce870a28a559f4e81dcfc54cbd128cb7)
Signed-off-by: Shivaram Venkataraman <ad63b47dfac1951bde2b661ba9fbbe123f686bee@cs.berkeley.edu>

ibm-research-ireland/sparkoscope,kimoonkim/spark,SnappyDataInc/spark,metamx/spark,SnappyDataInc/spark,Panos-Bletsos/spark-cost-model-optimizer,huang1900/spark,huang1900/spark,huang1900/spark,SnappyDataInc/spark,kimoonkim/spark,big-pegasus/spark,big-pegasus/spark,kimoonkim/spark,Panos-Bletsos/spark-cost-model-optimizer,SnappyDataInc/spark,metamx/spark,ibm-research-ireland/sparkoscope,SnappyDataInc/spark,metamx/spark,big-pegasus/spark,Panos-Bletsos/spark-cost-model-optimizer,Panos-Bletsos/spark-cost-model-optimizer,big-pegasus/spark,huang1900/spark,ibm-research-ireland/sparkoscope,big-pegasus/spark,SnappyDataInc/spark,SnappyDataInc/spark,metamx/spark,ibm-research-ireland/sparkoscope,Panos-Bletsos/spark-cost-model-optimizer,Panos-Bletsos/spark-cost-model-optimizer,kimoonkim/spark,Panos-Bletsos/spark-cost-model-optimizer,kimoonkim/spark,metamx/spark,big-pegasus/spark,huang1900/spark,ibm-research-ireland/sparkoscope,ibm-research-ireland/sparkoscope,kimoonkim/spark,metamx/spark,ibm-research-ireland/sparkoscope,metamx/spark,huang1900/spark,kimoonkim/spark,huang1900/spark,big-pegasus/spark