Updated mini-batch doc
Updated mini-batch doc


Former-commit-id: f9481829e80ffee418ec81ba9084286d3b67d9ab
xunzhang/dynet,shuheik/dynet,danielhers/dynet,xunzhang/dynet,chunyang-wen/dynet,clab/dynet,danielhers/dynet,danielhers/dynet,danielhers/dynet,danielhers/dynet,shuheik/dynet,chunyang-wen/dynet,clab/dynet,shuheik/dynet,clab/cnn,shuheik/dynet,chunyang-wen/dynet,clab/dynet,clab/dynet,clab/dynet,danielhers/dynet,clab/dynet,danielhers/dynet,clab/cnn,clab/dynet,chunyang-wen/dynet,clab/cnn,xunzhang/dynet,shuheik/dynet,xunzhang/dynet,shuheik/dynet,shuheik/dynet