Added hinge loss documentation
Added hinge loss documentation


Former-commit-id: 988dc7882934acb088866749c777031020f41270
chunyang-wen/dynet,xunzhang/dynet,clab/cnn,chunyang-wen/dynet,danielhers/dynet,shuheik/dynet,shuheik/dynet,danielhers/dynet,clab/dynet,clab/cnn,danielhers/dynet,shuheik/dynet,chunyang-wen/dynet,danielhers/dynet,danielhers/dynet,xunzhang/dynet,danielhers/dynet,shuheik/dynet,clab/dynet,chunyang-wen/dynet,shuheik/dynet,shuheik/dynet,clab/dynet,xunzhang/dynet,shuheik/dynet,clab/dynet,clab/dynet,xunzhang/dynet,clab/dynet,clab/cnn,danielhers/dynet,clab/dynet