Add const_parameter to python docs (#935)
Add const_parameter to python docs (#935)



Former-commit-id: 7770887e937d93b34272c68f61230c46cae6e36a
clab/dynet,chunyang-wen/dynet,danielhers/dynet,chunyang-wen/dynet,chunyang-wen/dynet,chunyang-wen/dynet,shuheik/dynet,clab/cnn,danielhers/dynet,danielhers/dynet,clab/dynet,xunzhang/dynet,shuheik/dynet,danielhers/dynet,xunzhang/dynet,clab/dynet,danielhers/dynet,shuheik/dynet,clab/dynet,shuheik/dynet,xunzhang/dynet,danielhers/dynet,clab/dynet,shuheik/dynet,clab/cnn,clab/dynet,xunzhang/dynet,clab/dynet,danielhers/dynet,shuheik/dynet,clab/cnn,shuheik/dynet