Cluster: redis-trib help output improved.

Show options if any. Clarify that for some command any node address is
ok.

src/redis-trib.rb

@@ -857,12 +857,18 @@ ALLOWED_OPTIONS={
 }
 
 def show_help
-    puts "Usage: redis-trib <command> <options> <arguments ...>"
-    puts
+    puts "Usage: redis-trib <command> <options> <arguments ...>\n\n"
     COMMANDS.each{|k,v|
-        puts "  #{k.ljust(10)} #{v[2]}"
+        o = ""
+        if ALLOWED_OPTIONS[k]
+            ALLOWED_OPTIONS[k].each{|optname,has_arg|
+                o += "--#{optname}" + (has_arg ? " <arg>" : "") + " "
             }
-    puts
+        end
+        o = "[#{o.strip}]" if o.length > 0
+        puts "  #{k.ljust(10)} #{v[2]} #{o}"
+    }
+    puts "\nFor check, fix, reshard, you can specify host:port of any working node.\n"
 end
 
 # Sanity check