[IR] Add inline docs to all statements (#2276)

* [ir] Add inline docs to statements

* add docs to all stmts

* Add "explicit" and "const &"

* [skip ci] enforce code format

* [skip ci] Apply review

* Apply suggestions from code review

Co-authored-by: Ye Kuang <k-ye@users.noreply.github.com>

* Apply review

* add TODO: remove InternalFuncStmt

Co-authored-by: Taichi Gardener <taichigardener@gmail.com>
Co-authored-by: Ye Kuang <k-ye@users.noreply.github.com>