FIx comment about diagnostic suppression.

Author: David Ungar

include/swift/AST/DiagnosticConsumer.h

@@ -188,8 +188,9 @@ class FileSpecificDiagnosticConsumer : public DiagnosticConsumer {
   /// None if diagnostic is to be distributed to every consumer,
   /// a particular consumer if diagnostic goes there.
   ///
-  /// (For Xcode compatibility, batch mode must suppress diagnostics to
-  /// non-primaries.)
+  /// (If batch mode failed to suppress diagnostics for non-primary files, it
+  /// would end up writing each of those diagnostics in each primary file's
+  /// serialized diagnostic file.)
   Optional<DiagnosticConsumer *> consumerForLocation(SourceManager &SM,
                                                      SourceLoc loc) const;
 };