Fix Split documentation
Fix Split documentation

Summary: Split doc failed to mention important features like specifying 'split' argument. Two questions the same day in Caffe2 Users were about how to do this.

Reviewed By: azzolini

Differential Revision: D5009503

fbshipit-source-id: 883549be891705a5c83778302d967481419f4dde

sf-wind/caffe2,sf-wind/caffe2,xzturn/caffe2,pietern/caffe2,pietern/caffe2,bwasti/caffe2,bwasti/caffe2,Yangqing/caffe2,caffe2/caffe2,davinwang/caffe2,pietern/caffe2,sf-wind/caffe2,Yangqing/caffe2,davinwang/caffe2,xzturn/caffe2,sf-wind/caffe2,pietern/caffe2,bwasti/caffe2,Yangqing/caffe2,xzturn/caffe2,xzturn/caffe2,bwasti/caffe2,davinwang/caffe2,bwasti/caffe2,Yangqing/caffe2,xzturn/caffe2,davinwang/caffe2,pietern/caffe2,sf-wind/caffe2,davinwang/caffe2,Yangqing/caffe2