Added support for help text to be shown on syntax errors and such.

src/main/java/com/sk89q/minecraft/util/commands/Command.java

@@ -67,4 +67,9 @@ public @interface Command {
      * meaning that if it is given it must have a value
      */
     String flags() default "";
+
+    /**
+     * A long description for the command.
+     */
+    String help() default "";
 }

src/main/java/com/sk89q/minecraft/util/commands/CommandsManager.java

@@ -242,7 +242,7 @@ public abstract class CommandsManager<T> {
      * @return
      */
     protected String getUsage(String[] args, int level, Command cmd) {
-        StringBuilder command = new StringBuilder();
+        final StringBuilder command = new StringBuilder();
 
         command.append('/');
 
@@ -262,6 +262,12 @@ public abstract class CommandsManager<T> {
         }
         command.append(cmd.usage());
 
+        final String help = cmd.help();
+        if (!help.isEmpty()) {
+            command.append("\n\n");
+            command.append(help);
+        }
+
         return command.toString();
     }