Fix the description on how to whitelist a protocol handler

Fixed #295

user.js

@@ -323,7 +323,9 @@ user_pref("browser.display.use_document_fonts",			0);
 // NOTICE: Disabling nonessential protocols breaks all interaction with custom protocols such as mailto:, irc:, magnet: ... and breaks opening third-party mail/messaging/torrent/... clients when clicking on links with these protocols
 // TODO: Add externally-handled protocols from Windows 8.1 and Windows 10 (currently contains protocols only from Linux and Windows 7) that might pose a similar threat (see e.g. https://news.ycombinator.com/item?id=13044991)
 // TODO: Add externally-handled protocols from Mac OS X that might pose a similar threat (see e.g. https://news.ycombinator.com/item?id=13044991)
-// If you want to enable a protocol, delete all three preferences for the protocol (external.protocolname, warn-external.protocolname, expose.protocolname).
+// If you want to enable a protocol, set network.protocol-handler.expose.(protocol) to true and network.protocol-handler.external.(protocol) to:
+//   * true, if the protocol should be handled by an external application
+//   * false, if the protocol should be handled internally by Firefox
 user_pref("network.protocol-handler.warn-external-default",	true);
 user_pref("network.protocol-handler.external.http",		false);
 user_pref("network.protocol-handler.external.https",		false);