AccumulableParam gave its generic parameters as 'R, T', whereas SparkContext labeled them 'T, R'.
Trivial, but really confusing.
I resolved this in favor of AccumulableParam, because it seemed to have some logic for its names. I also extended this minimal, but at least present, justification into the SparkContext comments.
Author: Nathan Kronenfeld <nkronenfeld@oculusinfo.com>
Closes#2637 from nkronenfeld/accumulators and squashes the following commits:
98d6b74 [Nathan Kronenfeld] Rectify gereneric parameter names between SparkContext and AccumulableParam
fd7b155396