Added documentation for some arithmetic
Added documentation for some arithmetic


Former-commit-id: 84ca6a2f821b4a9ab0dd4a5081e18f056761a37d
shuheik/dynet,shuheik/dynet,clab/dynet,clab/cnn,danielhers/dynet,chunyang-wen/dynet,shuheik/dynet,clab/dynet,danielhers/dynet,danielhers/dynet,clab/cnn,shuheik/dynet,danielhers/dynet,clab/dynet,clab/dynet,danielhers/dynet,chunyang-wen/dynet,shuheik/dynet,danielhers/dynet,clab/dynet,clab/dynet,shuheik/dynet,clab/dynet,xunzhang/dynet,danielhers/dynet,xunzhang/dynet,xunzhang/dynet,xunzhang/dynet,chunyang-wen/dynet,chunyang-wen/dynet,shuheik/dynet,clab/cnn