Bug 823023 - Tweak the comment at the top of prefs.js. r=glandium

Specifically:
- Make the warning about editing in all-caps;
- Make it clear that about:config is a browser thing;
- Add a mention of the user.js file;
- Use C++ comments, because I prefer them to C comments and I am the module
  owner :)

MozReview-Commit-ID: 9GXS5nNHywO

modules/libpref/Preferences.cpp

@@ -2344,23 +2344,24 @@ static const char kChannelPref[] = "app.update.channel";
 
 // clang-format off
 static const char kPrefFileHeader[] =
-  "# Mozilla User Preferences"
+  "// Mozilla User Preferences"
   NS_LINEBREAK
   NS_LINEBREAK
-  "/* Do not edit this file."
+  "// DO NOT EDIT THIS FILE."
   NS_LINEBREAK
-  " *"
+  "//"
   NS_LINEBREAK
-  " * If you make changes to this file while the application is running,"
+  "// If you make changes to this file while the application is running,"
   NS_LINEBREAK
-  " * the changes will be overwritten when the application exits."
+  "// the changes will be overwritten when the application exits."
   NS_LINEBREAK
-  " *"
+  "//"
   NS_LINEBREAK
-  " * To make a manual change to preferences, you can visit the URL "
-  "about:config"
+  "// To change a preference value, you can either:"
   NS_LINEBREAK
-  " */"
+  "// - modify it via the UI (e.g. via about:config in the browser); or"
+  NS_LINEBREAK
+  "// - set it within a user.js file in your profile."
   NS_LINEBREAK
   NS_LINEBREAK;
 // clang-format on