Rectify gereneric parameter names between SparkContext and AccumulablePa...

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.