add comment on limits to common_grammar_trigger.to/from json specializations

Author: Olivier Chafik

common/common.h

@@ -122,6 +122,7 @@ struct common_grammar_trigger {
     std::string value;
     llama_token token = LLAMA_TOKEN_NULL;
 
+    // T can only be nlohmann::ordered_json
     template <class T> T to_json() const;
     template <class T> static common_grammar_trigger from_json(const T & in);
 };