[Doc] Taichi spec: parenthesized forms; expression lists (#4653)
[Doc] Taichi spec: parenthesized forms; expression lists (#4653)

* [Doc] Taichi spec: parenthesized forms; expression lists

* Remove redundant space
yuanming-hu/taichi,yuanming-hu/taichi,yuanming-hu/taichi,yuanming-hu/taichi