Function match_arg_doc_placeholder_map 

pub(crate) fn match_arg_doc_placeholder_map(
    c_ident: &str,
    per_param: &HashMap<String, ParamAttrs>,
) -> 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.

Takes the per-param attribute map directly (rather than &ParsedMethod) so it’s shared by both the inherent-impl (MethodContext) and trait-impl (TraitMethodContext, miniextendr_impl_trait/method_context.rs) paths.