[SPARK-6824] Fill the docs for DataFrame API in SparkR

This patch also removes the RDD docs from being built as a part of roxygen just by the method to delete
" ' '" of " \#' ".

Author: hqzizania <qian.huang@intel.com>
Author: qhuang <qian.huang@intel.com>

Closes #5969 from hqzizania/R1 and squashes the following commits:

6d27696 [qhuang] fixes in NAMESPACE
eb4b095 [qhuang] remove more docs
6394579 [qhuang] remove RDD docs in generics.R
6813860 [hqzizania] Fill the docs for DataFrame API in SparkR
857220f [hqzizania] remove the pairRDD docs from being built as a part of roxygen
c045d64 [hqzizania] remove the RDD docs from being built as a part of roxygen