add even dim requirement and dimension splitting to BiRNN doc
add even dim requirement and dimension splitting to BiRNN doc


Former-commit-id: e62bc6a5599d164eddefe601c83ee862169506c6
xunzhang/dynet,xunzhang/dynet,shuheik/dynet,clab/dynet,clab/dynet,clab/cnn,chunyang-wen/dynet,chunyang-wen/dynet,shuheik/dynet,danielhers/dynet,shuheik/dynet,shuheik/dynet,shuheik/dynet,danielhers/dynet,danielhers/dynet,xunzhang/dynet,clab/dynet,shuheik/dynet,chunyang-wen/dynet,clab/cnn,danielhers/dynet,clab/cnn,clab/dynet,shuheik/dynet,danielhers/dynet,clab/dynet,danielhers/dynet,chunyang-wen/dynet,xunzhang/dynet,clab/dynet,clab/dynet,danielhers/dynet