Function match_arg_doc_placeholder_map 

pub(crate) fn match_arg_doc_placeholder_map(
    c_ident: &str,
    method: &ParsedMethod,
) -> HashMap<String, String>

Build the R-param-name → @param placeholder map for a method’s match_arg and choices params. Pass to MethodDocBuilder::with_match_arg_doc_placeholders in each class generator.