provide config instructions for MongoDB Atlas

server/tinode.conf

@@ -304,7 +304,12 @@
 			// MongoDB configuration.
 			"mongodb": {
 				// Connection string https://www.mongodb.com/docs/manual/reference/connection-string/
-				// Options configured with the connection string override any other options.
+				// Options configured with the connection string override any other options
+				// (only 'uri' is used to the server, all other options ignored).
+				// If you are using Atlas, then you MUST use 'uri' to connect. See here:
+				// https://www.mongodb.com/docs/manual/reference/connection-string/#std-label-connections-dns-seedlist
+				// Something like
+				// "uri": "mongodb+srv://CREDENTIALS@PROJECT.gmuaq.mongodb.net/DATABASE?retryWrites=true&w=majority",
 				"uri": "",
 				// The only supported server API version is "1". May or maynot be needed depending on server version.
 				"api_version": "",