Fix the comments for libsyntax::parse::parser::parse_sugary_call_expr

The comments on this function date back from when it was used for `for`
expressions in addition to `do` expressions.
auto merge of #10792 : kballard/rust/parse_sugary_call_expr-comments, r=alexcrichton

The comments on this function date back from when it was used for `for`
expressions in addition to `do` expressions.