Fixed comment for csharp_umbrella_namespace (nest_classes has been removed)

src/google/protobuf/descriptor.proto

@@ -356,8 +356,7 @@ message FileOptions {
 
   // A nested namespace for the umbrella class.  Helpful for name collisions caused by
   // umbrella_classname conflicting with an existing type.  This will be automatically
-  // set to 'Proto' if a collision is detected with types being generated.  This value
-  // is ignored when nest_classes == true
+  // set to 'Proto' if a collision is detected with types being generated.
   optional string csharp_umbrella_namespace = 38;
 
   // The parser stores options it doesn't recognize here. See above.