Update SoftmaxOp documentation: input not necessarily 2-D
Update SoftmaxOp documentation: input not necessarily 2-D

Summary: Update SoftmaxOp documentation: input not necessarily 2-D

Reviewed By: jamesr66a

Differential Revision: D5535238

fbshipit-source-id: 1beed3b35348737c51cd564d95e9f87e9ba0608a

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