Function parse_r_data_prop_doc 

fn parse_r_data_prop_doc(field: &Field) -> Result<Option<String>>

Parse prop_doc = "..." from an #[r_data(prop_doc = "...")] attribute.

Returns None if the field has no #[r_data] attribute, no prop_doc key, or if the attribute has no parenthesized arguments (bare #[r_data]).