add back comments for trailers

proto/store.proto

@@ -7,21 +7,34 @@ option csharp_namespace = "Momento.Protos.Store";
 
 package store;
 
-// Possible headers returned in grpc error responses
+// Found on the `err` trailer for error responses, when the error is sent by the server and it has a relevant value to set
 // {
 //   Key: "err"
 //   Values: [
+//      This is to indicate the error is coming from Momento, and not tonic or other middleware
 //    "momento_general_err",
+//      The server may or may not have tried to process this command, but it was unable
+//      to complete it either way. A resource was exhausted which prevented the request
+//      from completing.
 //    "server_is_busy",
+//      Indicates that the stored type for the key supplied isn't compatiable with the 
+//      requested operation
 //    "invalid_type",
+//      Indicates the item doesn't exist for the given key
 //    "item_not_found",
+//      Indicates the store doesn't exist
 //    "store_not_found"
 //   ]
 // },
+// Found on the `retry_disposition` trailer for error responses, when the value is known by the service
 // {
 //   Key: "retry_disposition"
 //   Values: [
+//      This rpc is safe to retry, even if it is non-idempotent. It was not executed by the server.
 //    "retryable",
+//      This rpc may be safe to retry, but it may have been applied.
+//      Non-idempotent commands should not be retried, unless configured explicitly.
+//      Idempotent commands should be considered eligible for retry.
 //    "possibly_retryable"
 //   ]
 // }