Re-add accidentally deleted doc comment.

sdk/lib/_internal/pub/lib/src/barback.dart

Index: sdk/lib/_internal/pub/lib/src/barback.dart
diff --git a/sdk/lib/_internal/pub/lib/src/barback.dart b/sdk/lib/_internal/pub/lib/src/barback.dart
index b0a5581a752395bb4d574d0673783bbc4606e065..5c99bfa06503de7eee9f432512bc5cd978a1c557 100644
--- a/sdk/lib/_internal/pub/lib/src/barback.dart
+++ b/sdk/lib/_internal/pub/lib/src/barback.dart
@@ -57,7 +57,9 @@ class TransformerId {
 
   /// The configuration to pass to the transformer.
   ///
-  /// This will be an empty map if no configuration was provided.
+  /// Any pub-specific configuration (i.e. keys starting with "$") will have
+  /// been stripped out of this and handled separately. This will be an empty
+  /// map if no configuration was provided.
   final Map configuration;
 
   /// The primary input inclusions.