Added add inputs documentation
Added add inputs documentation


Former-commit-id: a8dac4e611042ea8ed409b2757167ca573113c63
clab/cnn,chunyang-wen/dynet,clab/dynet,xunzhang/dynet,clab/dynet,chunyang-wen/dynet,danielhers/dynet,clab/dynet,clab/dynet,danielhers/dynet,chunyang-wen/dynet,shuheik/dynet,clab/dynet,shuheik/dynet,shuheik/dynet,clab/cnn,shuheik/dynet,danielhers/dynet,clab/cnn,danielhers/dynet,shuheik/dynet,clab/dynet,clab/dynet,xunzhang/dynet,danielhers/dynet,shuheik/dynet,danielhers/dynet,danielhers/dynet,chunyang-wen/dynet,xunzhang/dynet,xunzhang/dynet,shuheik/dynet