add some descriptions for Config.java

(cherry picked from commit b83185bbca)

src/main/java/eu/siacs/conversations/Config.java

@@ -216,6 +216,7 @@ public final class Config {
     }
 
     public static class DOMAIN {
+        // use this fallback server if provider list can't be updated automatically
         public static final List<String> DOMAINS = Arrays.asList(
                 "monocles.de",
                 "monocles.eu",
@@ -230,10 +231,12 @@ public final class Config {
                 "monocles.cn"
         );
 
+        // don't use these servers in provider list
         public static final List<String> BLACKLISTED_DOMAINS = Arrays.asList(
                 "blabber.im"
         );
 
+        // choose a random server for registration
         public static String getRandomServer() {
             try {
                 new ProviderService().execute();