Added documentation of losses
Added documentation of losses


Former-commit-id: 174c010c15e787584223c3e4b8f8c7be4c9edc13
shuheik/dynet,clab/cnn,clab/cnn,shuheik/dynet,shuheik/dynet,chunyang-wen/dynet,danielhers/dynet,xunzhang/dynet,clab/dynet,clab/dynet,clab/dynet,shuheik/dynet,xunzhang/dynet,chunyang-wen/dynet,clab/dynet,danielhers/dynet,shuheik/dynet,clab/dynet,xunzhang/dynet,danielhers/dynet,chunyang-wen/dynet,xunzhang/dynet,clab/dynet,danielhers/dynet,danielhers/dynet,danielhers/dynet,chunyang-wen/dynet,shuheik/dynet,danielhers/dynet,shuheik/dynet,clab/dynet,clab/cnn